diff --git a/Steamless.API/PE64/Pe64Helpers.cs b/Steamless.API/PE64/Pe64Helpers.cs
index ee98c7e..d3e8289 100644
--- a/Steamless.API/PE64/Pe64Helpers.cs
+++ b/Steamless.API/PE64/Pe64Helpers.cs
@@ -27,6 +27,7 @@ namespace Steamless.API.PE64
 {
     using System;
     using System.Collections.Generic;
+    using System.IO;
     using System.Linq;
     using System.Runtime.InteropServices;
 
@@ -89,6 +90,97 @@ namespace Steamless.API.PE64
             return GetStructure<NativeApi64.ImageSectionHeader64>(rawData, (int)dataOffset + (index * sectionSize));
         }
 
+        /// <summary>
+        /// Calculates the PE checksum of the opened file. (OptionalHeader.Checksum should be 0 before calling this!)
+        /// </summary>
+        /// <param name="fStream"></param>
+        /// <returns></returns>
+        private static byte[] CalculateChecksum(FileStream fStream)
+        {
+            fStream.Position = 0;
+
+            var dataSize = fStream.Length;
+            var totalWords = dataSize / 2;
+            var sumTotal = 0;
+
+            // Process the file data in uint16_t chunks..
+            while (totalWords > 0)
+            {
+                var sumChunk = 0;
+                var chunkWords = totalWords;
+
+                // Prepare next chunk size..
+                if (chunkWords > UInt16.MaxValue)
+                    chunkWords = UInt16.MaxValue;
+
+                totalWords -= chunkWords;
+
+                do
+                {
+                    var data = new byte[2];
+                    fStream.Read(data, 0, 2);
+                    sumChunk += BitConverter.ToUInt16(data, 0);
+                } while (--chunkWords != 0);
+
+                sumTotal += (sumChunk >> 16) + (sumChunk & 0xFFFF);
+            }
+
+            if ((dataSize % 2) != 0)
+                sumTotal += fStream.ReadByte();
+
+            var checksum = (uint)(((sumTotal >> 16) + (sumTotal & 0xFFFF)) + dataSize);
+
+            return BitConverter.GetBytes(checksum);
+        }
+
+        /// <summary>
+        /// Updates the given files PE checksum value. (Path is assumed to be a 32bit PE file.)
+        /// </summary>
+        /// <param name="path"></param>
+        /// <returns></returns>
+        public static bool UpdateFileChecksum(string path)
+        {
+            FileStream fStream = null;
+            var data = new byte[4];
+
+            try
+            {
+                // Open the file for reading/writing..
+                fStream = new FileStream(path, FileMode.Open, FileAccess.ReadWrite);
+
+                // Read the starting offset to the files NT headers..
+                fStream.Position = (int)Marshal.OffsetOf(typeof(NativeApi64.ImageDosHeader64), "e_lfanew");
+                fStream.Read(data, 0, 4);
+
+                var offset = BitConverter.ToUInt32(data, 0);
+
+                // Move to the files CheckSum position..
+                offset += 4 + (uint)Marshal.SizeOf(typeof(NativeApi64.ImageFileHeader64)) + (uint)Marshal.OffsetOf(typeof(NativeApi64.ImageOptionalHeader64), "CheckSum").ToInt32();
+                fStream.Position = offset;
+
+                // Ensure the checksum is 0 to start..
+                data = new byte[4] { 0, 0, 0, 0 };
+                fStream.Write(data, 0, 4);
+
+                // Calculate the new checksum..
+                var checksum = CalculateChecksum(fStream);
+
+                // Update the checksum value..
+                fStream.Position = offset;
+                fStream.Write(checksum, 0, 4);
+
+                return true;
+            }
+            catch
+            {
+                return false;
+            }
+            finally
+            {
+                fStream?.Dispose();
+            }
+        }
+
         /// <summary>
         /// Scans the given data for the given pattern.
         /// 
diff --git a/Steamless.Unpacker.Variant30.x64/Main.cs b/Steamless.Unpacker.Variant30.x64/Main.cs
index 6d96bd5..e52c858 100644
--- a/Steamless.Unpacker.Variant30.x64/Main.cs
+++ b/Steamless.Unpacker.Variant30.x64/Main.cs
@@ -189,6 +189,13 @@ namespace Steamless.Unpacker.Variant30.x64
             if (!this.Step6())
                 return false;
 
+            if (this.Options.RecalculateFileChecksum)
+            {
+                this.Log("Step 7 - Rebuild unpacked file checksum.", LogMessageType.Information);
+                if (!this.Step7())
+                    return false;
+            }
+
             return true;
         }
 
@@ -500,12 +507,13 @@ namespace Steamless.Unpacker.Variant30.x64
                 if (this.File.DosStubSize > 0)
                     fStream.WriteBytes(this.File.DosStubData);
 
-                // Update the entry point of the file..
+                // Update the NT headers..
                 var ntHeaders = this.File.NtHeaders;
                 if (this.StubHeader.HasTlsCallback != 1)
                     ntHeaders.OptionalHeader.AddressOfEntryPoint = this.StubHeader.OriginalEntryPoint;
                 else
                     ntHeaders.OptionalHeader.AddressOfEntryPoint = this.TlsOepOverride;
+                ntHeaders.OptionalHeader.CheckSum = 0;
                 this.File.NtHeaders = ntHeaders;
 
                 // Write the NT headers to the file..
@@ -558,6 +566,26 @@ namespace Steamless.Unpacker.Variant30.x64
             }
         }
 
+        /// <summary>
+        /// Step #7
+        /// 
+        /// Recalculate the file checksum.
+        /// </summary>
+        /// <returns></returns>
+        private bool Step7()
+        {
+            var unpackedPath = this.File.FilePath + ".unpacked.exe";
+            if (!Pe64Helpers.UpdateFileChecksum(unpackedPath))
+            {
+                this.Log(" --> Error trying to recalculate unpacked file checksum!", LogMessageType.Error);
+                return false;
+            }
+
+            this.Log(" --> Unpacked file updated with new checksum!", LogMessageType.Success);
+            return true;
+
+        }
+
         /// <summary>
         /// Gets or sets if the Tls callback is being used as the Oep.
         /// </summary>
diff --git a/Steamless.Unpacker.Variant31.x64/Main.cs b/Steamless.Unpacker.Variant31.x64/Main.cs
index bd08f96..da09cb9 100644
--- a/Steamless.Unpacker.Variant31.x64/Main.cs
+++ b/Steamless.Unpacker.Variant31.x64/Main.cs
@@ -186,6 +186,13 @@ namespace Steamless.Unpacker.Variant31.x64
             if (!this.Step6())
                 return false;
 
+            if (this.Options.RecalculateFileChecksum)
+            {
+                this.Log("Step 7 - Rebuild unpacked file checksum.", LogMessageType.Information);
+                if (!this.Step7())
+                    return false;
+            }
+
             return true;
         }
 
@@ -452,9 +459,10 @@ namespace Steamless.Unpacker.Variant31.x64
                 if (this.File.DosStubSize > 0)
                     fStream.WriteBytes(this.File.DosStubData);
 
-                // Update the entry point of the file..
+                // Update the NT headers..
                 var ntHeaders = this.File.NtHeaders;
                 ntHeaders.OptionalHeader.AddressOfEntryPoint = (uint)this.StubHeader.OriginalEntryPoint;
+                ntHeaders.OptionalHeader.CheckSum = 0;
                 this.File.NtHeaders = ntHeaders;
 
                 // Write the NT headers to the file..
@@ -507,6 +515,26 @@ namespace Steamless.Unpacker.Variant31.x64
             }
         }
 
+        /// <summary>
+        /// Step #7
+        /// 
+        /// Recalculate the file checksum.
+        /// </summary>
+        /// <returns></returns>
+        private bool Step7()
+        {
+            var unpackedPath = this.File.FilePath + ".unpacked.exe";
+            if (!Pe64Helpers.UpdateFileChecksum(unpackedPath))
+            {
+                this.Log(" --> Error trying to recalculate unpacked file checksum!", LogMessageType.Error);
+                return false;
+            }
+
+            this.Log(" --> Unpacked file updated with new checksum!", LogMessageType.Success);
+            return true;
+
+        }
+
         /// <summary>
         /// Gets or sets if the Tls callback is being used as the Oep.
         /// </summary>