diff --git a/guix/ftp-client.scm b/guix/ftp-client.scm
index 7e241f37b2..e3bacc3720 100644
--- a/guix/ftp-client.scm
+++ b/guix/ftp-client.scm
@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2010, 2011, 2012 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2010, 2011, 2012, 2013 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -33,6 +33,7 @@ (define-module (guix ftp-client)
             ftp-open
             ftp-close
             ftp-chdir
+            ftp-size
             ftp-list
             ftp-retr))
 
@@ -133,6 +134,12 @@ (define (ftp-chdir conn dir)
   (%ftp-command (string-append "CWD " dir) 250
                 (ftp-connection-socket conn)))
 
+(define (ftp-size conn file)
+  "Return the size in bytes of FILE."
+  (let ((message (%ftp-command (string-append "SIZE " file) 213
+                               (ftp-connection-socket conn))))
+    (string->number (string-trim-both message))))
+
 (define (ftp-pasv conn)
   (define %pasv-rx
     (make-regexp "([0-9]+),([0-9]+),([0-9]+),([0-9]+),([0-9]+),([0-9]+)"))