diff --git a/mumi/client.scm b/mumi/client.scm
index 2750836..f0e4321 100644
--- a/mumi/client.scm
+++ b/mumi/client.scm
@@ -18,6 +18,7 @@
 
 (define-module (mumi client)
   #:use-module (rnrs io ports)
+  #:use-module (srfi srfi-1)
   #:use-module (srfi srfi-19)
   #:use-module (srfi srfi-26)
   #:use-module (srfi srfi-43)
@@ -236,15 +237,41 @@ OPTIONS. Return the message ID of the first email sent."
           (display (get-string-all port))
           message-id)))))
 
+(define (reply-email-headers issue-number)
+  "Return an association list of email headers when replying to
+ISSUE-NUMBER."
+  (let ((messages
+         (assoc-ref
+          (assoc-ref
+           (graphql-http-get (graphql-endpoint)
+                             `(document
+                               (query (#(issue #:number ,issue-number)
+                                       (messages (from name address)
+                                                 date)))))
+           "issue")
+          "messages")))
+    ;; When sending email to an issue, we Cc all issue participants.
+    ;; TODO: Also add an In-Reply-To header.
+    `((cc . ,(delete-duplicates
+              (map (lambda (message)
+                     (let ((from (assoc-ref message "from")))
+                       (string-append (assoc-ref from "name")
+                                      " <" (assoc-ref from "address") ">")))
+                   (vector->list messages)))))))
+
 (define (send-email patches)
   "Send PATCHES via email."
   (if (current-issue-number)
       ;; If an issue is current, send patches to that issue's email
       ;; address.
-      (git-send-email (string-append (number->string (current-issue-number))
-                                     "@"
-                                     (client-config 'debbugs-host))
-                      patches)
+      (let ((issue-number (current-issue-number)))
+        (git-send-email (string-append (number->string issue-number)
+                                       "@"
+                                       (client-config 'debbugs-host))
+                        patches
+                        (map (cut string-append "--cc=" <>)
+                             (assq-ref (reply-email-headers issue-number)
+                                       'cc))))
       (match patches
         ;; If it's a single patch, send it to the patch email address
         ;; and be done with it
diff --git a/tests/client.scm b/tests/client.scm
index 94c8c5d..f0ff34e 100644
--- a/tests/client.scm
+++ b/tests/client.scm
@@ -90,6 +90,8 @@ called with."
          (lambda ()
            (with-variables (list (cons (var@@ (mumi client) current-issue-number)
                                        (const 12345))
+                                 (cons (var@@ (mumi client) reply-email-headers)
+                                       (const '((cc))))
                                  client-config-stub
                                  do-not-poll-server-for-issue-number)
              (cut (@@ (mumi client) send-email)
@@ -116,6 +118,28 @@ called with."
          (lambda ()
            (with-variables (list (cons (var@@ (mumi client) current-issue-number)
                                        (const 12345))
+                                 (cons (var@@ (mumi client) reply-email-headers)
+                                       (const '((cc))))
+                                 client-config-stub
+                                 do-not-poll-server-for-issue-number)
+             (cut (@@ (mumi client) send-email)
+                  (list "foo.patch")))))))
+
+(test-equal "send patch to existing issue and Cc other participants"
+  '(("git" "send-email"
+     "--to=12345@example.com"
+     "--cc=John Doe <jdoe@machine.example>"
+     "--cc=Mary Smith <mary@example.net>"
+     "foo.patch"))
+  (map (match-lambda
+         ((command _) command))
+       (trace-calls (var@@ (mumi client) call-with-input-pipe)
+         (lambda ()
+           (with-variables (list (cons (var@@ (mumi client) current-issue-number)
+                                       (const 12345))
+                                 (cons (var@@ (mumi client) reply-email-headers)
+                                       (const `((cc "John Doe <jdoe@machine.example>"
+                                                    "Mary Smith <mary@example.net>"))))
                                  client-config-stub
                                  do-not-poll-server-for-issue-number)
              (cut (@@ (mumi client) send-email)
