diff --git a/src/lib.rs b/src/lib.rs
index 0ff18f97..a2d3d189 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -28,7 +28,7 @@ use std::ops::{Deref, DerefMut};
 /// `MmapOptions::map_exec`, or `MmapOptions::map_copy`.
 #[derive(Clone, Debug, Default)]
 pub struct MmapOptions {
-    offset: usize,
+    offset: u64,
     len: Option<usize>,
     stack: bool,
 }
@@ -86,7 +86,7 @@ impl MmapOptions {
     /// # }
     /// # fn main() { try_main().unwrap(); }
     /// ```
-    pub fn offset(&mut self, offset: usize) -> &mut Self {
+    pub fn offset(&mut self, offset: u64) -> &mut Self {
         self.offset = offset;
         self
     }
@@ -123,13 +123,14 @@ impl MmapOptions {
     fn get_len(&self, file: &File) -> Result<usize> {
         self.len.map(Ok).unwrap_or_else(|| {
             let len = file.metadata()?.len();
+            let len = len - self.offset;
             if len > (usize::MAX as u64) {
                 return Err(Error::new(
                     ErrorKind::InvalidData,
                     "file length overflows usize",
                 ));
             }
-            Ok(len as usize - self.offset)
+            Ok(len as usize)
         })
     }
 
@@ -829,12 +830,20 @@ mod test {
             .open(&path)
             .unwrap();
 
-        file.set_len(500000 as u64).unwrap();
+        let offset = u32::max_value() as u64 + 2;
+        let len = 5432;
+        file.set_len(offset + len as u64).unwrap();
 
-        let offset = 5099;
-        let len = 50050;
 
+        // Test both auto-length and explicit length
         let mut mmap = unsafe {
+            MmapOptions::new()
+                .offset(offset)
+                .map_mut(&file)
+                .unwrap()
+        };
+        assert_eq!(len, mmap.len());
+        mmap = unsafe {
             MmapOptions::new()
                 .offset(offset)
                 .len(len)
diff --git a/src/unix.rs b/src/unix.rs
index fbece88f..0abaa32d 100644
--- a/src/unix.rs
+++ b/src/unix.rs
@@ -26,10 +26,10 @@ impl MmapInner {
         prot: libc::c_int,
         flags: libc::c_int,
         file: RawFd,
-        offset: usize,
+        offset: u64,
     ) -> io::Result<MmapInner> {
-        let alignment = offset % page_size();
-        let aligned_offset = offset - alignment;
+        let alignment = (offset % page_size() as u64) as usize;
+        let aligned_offset = offset - alignment as u64;
         let aligned_len = len + alignment;
         if aligned_len == 0 {
             // Normally the OS would catch this, but it segfaults under QEMU.
@@ -60,7 +60,7 @@ impl MmapInner {
         }
     }
 
-    pub fn map(len: usize, file: &File, offset: usize) -> io::Result<MmapInner> {
+    pub fn map(len: usize, file: &File, offset: u64) -> io::Result<MmapInner> {
         MmapInner::new(
             len,
             libc::PROT_READ,
@@ -70,7 +70,7 @@ impl MmapInner {
         )
     }
 
-    pub fn map_exec(len: usize, file: &File, offset: usize) -> io::Result<MmapInner> {
+    pub fn map_exec(len: usize, file: &File, offset: u64) -> io::Result<MmapInner> {
         MmapInner::new(
             len,
             libc::PROT_READ | libc::PROT_EXEC,
@@ -80,7 +80,7 @@ impl MmapInner {
         )
     }
 
-    pub fn map_mut(len: usize, file: &File, offset: usize) -> io::Result<MmapInner> {
+    pub fn map_mut(len: usize, file: &File, offset: u64) -> io::Result<MmapInner> {
         MmapInner::new(
             len,
             libc::PROT_READ | libc::PROT_WRITE,
@@ -90,7 +90,7 @@ impl MmapInner {
         )
     }
 
-    pub fn map_copy(len: usize, file: &File, offset: usize) -> io::Result<MmapInner> {
+    pub fn map_copy(len: usize, file: &File, offset: u64) -> io::Result<MmapInner> {
         MmapInner::new(
             len,
             libc::PROT_READ | libc::PROT_WRITE,
diff --git a/src/windows.rs b/src/windows.rs
index a6f4bcdf..476241a2 100644
--- a/src/windows.rs
+++ b/src/windows.rs
@@ -46,12 +46,12 @@ impl MmapInner {
         file: &File,
         protect: DWORD,
         access: DWORD,
-        offset: usize,
+        offset: u64,
         len: usize,
         copy: bool,
     ) -> io::Result<MmapInner> {
-        let alignment = offset % allocation_granularity();
-        let aligned_offset = offset - alignment;
+        let alignment = (offset % allocation_granularity() as u64) as usize;
+        let aligned_offset = offset - alignment as u64;
         let aligned_len = len + alignment;
 
         unsafe {
@@ -70,7 +70,7 @@ impl MmapInner {
             let ptr = MapViewOfFile(
                 handle,
                 access,
-                (aligned_offset >> 16 >> 16) as DWORD,
+                (aligned_offset >> 32) as DWORD,
                 (aligned_offset & 0xffffffff) as DWORD,
                 aligned_len as SIZE_T,
             );
@@ -89,7 +89,7 @@ impl MmapInner {
         }
     }
 
-    pub fn map(len: usize, file: &File, offset: usize) -> io::Result<MmapInner> {
+    pub fn map(len: usize, file: &File, offset: u64) -> io::Result<MmapInner> {
         let write = protection_supported(file.as_raw_handle(), PAGE_READWRITE);
         let exec = protection_supported(file.as_raw_handle(), PAGE_EXECUTE_READ);
         let mut access = FILE_MAP_READ;
@@ -116,7 +116,7 @@ impl MmapInner {
         Ok(inner)
     }
 
-    pub fn map_exec(len: usize, file: &File, offset: usize) -> io::Result<MmapInner> {
+    pub fn map_exec(len: usize, file: &File, offset: u64) -> io::Result<MmapInner> {
         let write = protection_supported(file.as_raw_handle(), PAGE_READWRITE);
         let mut access = FILE_MAP_READ | FILE_MAP_EXECUTE;
         let protection = if write {
@@ -133,7 +133,7 @@ impl MmapInner {
         Ok(inner)
     }
 
-    pub fn map_mut(len: usize, file: &File, offset: usize) -> io::Result<MmapInner> {
+    pub fn map_mut(len: usize, file: &File, offset: u64) -> io::Result<MmapInner> {
         let exec = protection_supported(file.as_raw_handle(), PAGE_EXECUTE_READ);
         let mut access = FILE_MAP_READ | FILE_MAP_WRITE;
         let protection = if exec {
@@ -150,7 +150,7 @@ impl MmapInner {
         Ok(inner)
     }
 
-    pub fn map_copy(len: usize, file: &File, offset: usize) -> io::Result<MmapInner> {
+    pub fn map_copy(len: usize, file: &File, offset: u64) -> io::Result<MmapInner> {
         let exec = protection_supported(file.as_raw_handle(), PAGE_EXECUTE_READWRITE);
         let mut access = FILE_MAP_COPY;
         let protection = if exec {