diff --git a/autoload/asyncomplete.vim b/autoload/asyncomplete.vim
index 8cbf30f..9a31372 100644
--- a/autoload/asyncomplete.vim
+++ b/autoload/asyncomplete.vim
@@ -367,14 +367,21 @@ function! s:recompute_pum(...) abort
         endif
     endfor
 
-    " TODO: allow users to pass custom filter function. lock the api before making this public.
-    " Everything in this function should be treated as immutable. filter function shouldn't mutate.
-    call s:default_filter({ 'ctx': l:ctx, 'base': l:base, 'startcol': l:startcol, 'matches': l:matches_to_filter })
+    let l:filter_ctx = extend({
+        \ 'base': l:base,
+        \ 'startcol': l:startcol,
+        \ }, l:ctx)
+
+    if empty(g:asyncomplete_preprocessor)
+        call s:default_preprocessor(l:filter_ctx, l:matches_to_filter)
+    else
+        call g:asyncomplete_preprocessor[0](l:filter_ctx, l:matches_to_filter)
+    endif
 endfunction
 
-function! s:default_filter(options) abort
+function! s:default_preprocessor(options, matches) abort
     let l:items = []
-    for [l:source_name, l:matches] in items(a:options['matches'])
+    for [l:source_name, l:matches] in items(a:matches)
         for l:item in l:matches['items']
             if l:item['word'] =~ '^' . a:options['base']
                 call add(l:items, l:item)
@@ -382,17 +389,17 @@ function! s:default_filter(options) abort
         endfor
     endfor
 
-    call s:set_pum(a:options['startcol'], l:items)
+    call asyncomplete#preprocess_complete(a:options, l:items)
 endfunction
 
-function! s:set_pum(startcol, items) abort
-    " TODO: handle cases where this is called asynchronsouly
+function! asyncomplete#preprocess_complete(ctx, items)
+    " TODO: handle cases where this is called asynchronsouly. Currently not supported
     if s:should_skip() | return | endif
 
-    call asyncomplete#log('core', 's:set_pum')
+    call asyncomplete#log('core', 'asyncomplete#preprocess_complete')
 
     if asyncomplete#menu_selected()
-        call asyncomplete#log('core', 's:set_pum', 'ignorning set pum due to menu selection')
+        call asyncomplete#log('core', 'asyncomplete#preprocess_complete', 'ignorning pum update due to menu selection')
         return
     endif
 
@@ -400,8 +407,8 @@ function! s:set_pum(startcol, items) abort
         setl completeopt=menuone,noinsert,noselect
     endif
 
-    call asyncomplete#log('core', 's:set_pum calling complete()', a:startcol + 1, a:items)
-    call complete(a:startcol + 1, a:items)
+    call asyncomplete#log('core', 'asyncomplete#preprocess_complete calling complete()', a:ctx['startcol'] + 1, a:items)
+    call complete(a:ctx['startcol'] + 1, a:items)
 endfunction
 
 function! asyncomplete#menu_selected() abort
diff --git a/doc/asyncomplete.txt b/doc/asyncomplete.txt
index 48ddc72..0137be3 100644
--- a/doc/asyncomplete.txt
+++ b/doc/asyncomplete.txt
@@ -42,7 +42,7 @@ g:asyncomplete_popup_delay                         *g:asyncomplete_popup_delay*
 
     Milliseconds to wait before opening the popup menu
 
-g:asyncomplete_auto_completeopt                       *g:asyncomplete_auto_completeopt*
+g:asyncomplete_auto_completeopt               *g:asyncomplete_auto_completeopt*
 
     Type: |Number|
     Default: 1
@@ -52,6 +52,39 @@ g:asyncomplete_auto_completeopt                       *g:asyncomplete_auto_compl
 
     Set to 0 to disable.
 
+g:asyncomplete_preprocessor                        *g:asyncomplete_preprocessor*
+
+    Type: |Array| for zero or one |Function|
+    Default: []
+
+    Set a function to allow custom filtering or sorting.
+    Below example implements removing duplicates.
+
+      function! s:my_asyncomplete_preprocessor(ctx, matches) abort
+        let l:dict = {}
+        let l:items = []
+        for [l:source_name, l:matches] in items(a:matches)
+          for l:item in l:matches['items']
+            if l:item['word'] =~ '^' . a:options['base']
+              if !has_key(l:dict, l:item['word'])
+                call add(l:items, l:item)
+              endif
+            endif
+          endfor
+        endfor
+
+        call asyncomplete#preprocess_complete(a:ctx, l:items)
+      endfunction
+
+      let g:asyncomplete_preprocessor = [function('s:my_asyncomplete_preprocessor')]
+
+    Note:
+    asyncomplete#preprocess_complete() must be called synchronously.
+    Plans to support async preprocessing will be supported in the future.
+
+    context and matches in arguments in preprecessor function should be treated
+    as immutable.
+
 ===============================================================================
 3. Global vim configuration                          *asyncomplete-global-config*
 
diff --git a/plugin/asyncomplete.vim b/plugin/asyncomplete.vim
index 97b2648..8918050 100644
--- a/plugin/asyncomplete.vim
+++ b/plugin/asyncomplete.vim
@@ -18,5 +18,6 @@ let g:asyncomplete_auto_completeopt = get(g:, 'asyncomplete_auto_completeopt', 1
 let g:asyncomplete_auto_popup = get(g:, 'asyncomplete_auto_popup', 1)
 let g:asyncomplete_popup_delay = get(g:, 'asyncomplete_popup_delay', 30)
 let g:asyncomplete_log_file = get(g:, 'asyncomplete_log_file', '')
+let g:asyncomplete_preprocessor = get(g:, 'asyncomplete_preprocessor', [])
 
 inoremap <silent> <expr> <Plug>(asyncomplete_force_refresh) asyncomplete#force_refresh()