diff --git a/autoload/diff.vim b/autoload/diff.vim
index 54d825e..e08b5bd 100644
--- a/autoload/diff.vim
+++ b/autoload/diff.vim
@@ -7,7 +7,7 @@ function! diff#run_diff(realtime, use_external_grep)
   let cmd = 'git ls-files --error-unmatch ' . shellescape(utility#file()) . ' && ('
 
   if a:realtime
-    let blob_name = ':./' . fnamemodify(utility#file(),':t')
+    let blob_name = ':' . utility#file_relative_to_repo_root()
     let blob_file = tempname()
     let cmd .= 'git show ' . blob_name . ' > ' . blob_file .
           \ ' && diff -U0 ' . g:gitgutter_diff_args . ' ' . blob_file . ' - '
diff --git a/autoload/utility.vim b/autoload/utility.vim
index e864512..0b51aa6 100644
--- a/autoload/utility.vim
+++ b/autoload/utility.vim
@@ -4,6 +4,10 @@ function! utility#is_active()
   return g:gitgutter_enabled && utility#exists_file()
 endfunction
 
+function! utility#slash()
+  return !exists("+shellslash") || &shellslash ? '/' : '\'
+endfunction
+
 function! utility#current_file()
   return expand('%:p')
 endfunction
@@ -47,6 +51,16 @@ function! utility#buffer_contents()
   return join(getbufline(s:file, 1, '$'), eol) . eol
 endfunction
 
+function! utility#file_relative_to_repo_root()
+  let repo_root_for_file = getbufvar(s:file, 'gitgutter_repo_root')
+  if empty(repo_root_for_file)
+    let dir = system(utility#command_in_directory_of_file('git rev-parse --show-toplevel'))
+    let repo_root_for_file = substitute(dir, '\n$', '', '') . utility#slash()
+    call setbufvar(s:file, 'gitgutter_repo_root', repo_root_for_file)
+  endif
+  return substitute(s:file, repo_root_for_file, '', '')
+endfunction
+
 " https://github.com/tpope/vim-dispatch/blob/9cdd05a87f8a47120335be03dfcd8358544221cd/autoload/dispatch/windows.vim#L8-L17
 function! utility#escape(str)
   if &shellxquote ==# '"'