diff --git a/downstream.go b/downstream.go
index 5de6f8a..b39464b 100644
--- a/downstream.go
+++ b/downstream.go
@@ -945,6 +945,44 @@ func (dc *downstreamConn) handleMessageRegistered(msg *irc.Message) error {
 			Command: "WHO",
 			Params:  params,
 		})
+	case "WHOIS":
+		if len(msg.Params) == 0 {
+			return ircError{&irc.Message{
+				Command: irc.ERR_NONICKNAMEGIVEN,
+				Params:  []string{dc.nick, "No nickname given"},
+			}}
+		}
+
+		var target, mask string
+		if len(msg.Params) == 1 {
+			target = ""
+			mask = msg.Params[0]
+		} else {
+			target = msg.Params[0]
+			mask = msg.Params[1]
+		}
+		// TODO: support multiple WHOIS users
+		if i := strings.IndexByte(mask, ','); i >= 0 {
+			mask = mask[:i]
+		}
+
+		// TODO: support WHOIS masks
+		uc, upstreamNick, err := dc.unmarshalEntity(mask)
+		if err != nil {
+			return err
+		}
+
+		var params []string
+		if target != "" {
+			params = []string{target, upstreamNick}
+		} else {
+			params = []string{upstreamNick}
+		}
+
+		uc.SendMessage(&irc.Message{
+			Command: "WHOIS",
+			Params:  params,
+		})
 	case "PRIVMSG":
 		var targetsStr, text string
 		if err := parseMessageParams(msg, &targetsStr, &text); err != nil {
diff --git a/irc.go b/irc.go
index 17e0011..8df3a2a 100644
--- a/irc.go
+++ b/irc.go
@@ -86,6 +86,13 @@ const (
 
 const stdMembershipPrefixes = "~&@%+"
 
+func (m membership) String() string {
+	if m == 0 {
+		return ""
+	}
+	return string(m)
+}
+
 func parseMembershipPrefix(s string) (prefix membership, nick string) {
 	// TODO: any prefix from PREFIX RPL_ISUPPORT
 	if strings.IndexByte(stdMembershipPrefixes, s[0]) >= 0 {
diff --git a/upstream.go b/upstream.go
index c0e3868..bef66e2 100644
--- a/upstream.go
+++ b/upstream.go
@@ -603,7 +603,101 @@ func (uc *upstreamConn) handleMessage(msg *irc.Message) error {
 			dc.SendMessage(&irc.Message{
 				Prefix:  dc.srv.prefix(),
 				Command: irc.RPL_ENDOFWHO,
-				Params:  []string{dc.nick, name, "End of /WHO list."},
+				Params:  []string{dc.nick, name, "End of WHO list"},
+			})
+		})
+	case irc.RPL_WHOISUSER:
+		var nick, username, host, realname string
+		if err := parseMessageParams(msg, nil, &nick, &username, &host, nil, &realname); err != nil {
+			return err
+		}
+
+		uc.forEachDownstream(func(dc *downstreamConn) {
+			nick := dc.marshalNick(uc, nick)
+			dc.SendMessage(&irc.Message{
+				Prefix:  dc.srv.prefix(),
+				Command: irc.RPL_WHOISUSER,
+				Params:  []string{dc.nick, nick, username, host, "*", realname},
+			})
+		})
+	case irc.RPL_WHOISSERVER:
+		var nick, server, serverInfo string
+		if err := parseMessageParams(msg, nil, &nick, &server, &serverInfo); err != nil {
+			return err
+		}
+
+		uc.forEachDownstream(func(dc *downstreamConn) {
+			nick := dc.marshalNick(uc, nick)
+			dc.SendMessage(&irc.Message{
+				Prefix:  dc.srv.prefix(),
+				Command: irc.RPL_WHOISSERVER,
+				Params:  []string{dc.nick, nick, server, serverInfo},
+			})
+		})
+	case irc.RPL_WHOISOPERATOR:
+		var nick string
+		if err := parseMessageParams(msg, nil, &nick); err != nil {
+			return err
+		}
+
+		uc.forEachDownstream(func(dc *downstreamConn) {
+			nick := dc.marshalNick(uc, nick)
+			dc.SendMessage(&irc.Message{
+				Prefix:  dc.srv.prefix(),
+				Command: irc.RPL_WHOISOPERATOR,
+				Params:  []string{dc.nick, nick, "is an IRC operator"},
+			})
+		})
+	case irc.RPL_WHOISIDLE:
+		var nick string
+		if err := parseMessageParams(msg, nil, &nick, nil); err != nil {
+			return err
+		}
+
+		uc.forEachDownstream(func(dc *downstreamConn) {
+			nick := dc.marshalNick(uc, nick)
+			params := []string{dc.nick, nick}
+			params = append(params, msg.Params[2:]...)
+			dc.SendMessage(&irc.Message{
+				Prefix:  dc.srv.prefix(),
+				Command: irc.RPL_WHOISIDLE,
+				Params:  params,
+			})
+		})
+	case irc.RPL_WHOISCHANNELS:
+		var nick, channelList string
+		if err := parseMessageParams(msg, nil, &nick, &channelList); err != nil {
+			return err
+		}
+		channels := strings.Split(channelList, " ")
+
+		uc.forEachDownstream(func(dc *downstreamConn) {
+			nick := dc.marshalNick(uc, nick)
+			channelList := make([]string, len(channels))
+			for i, channel := range channels {
+				prefix, channel := parseMembershipPrefix(channel)
+				channel = dc.marshalChannel(uc, channel)
+				channelList[i] = prefix.String() + channel
+			}
+			channels := strings.Join(channelList, " ")
+			dc.SendMessage(&irc.Message{
+				Prefix:  dc.srv.prefix(),
+				Command: irc.RPL_WHOISCHANNELS,
+				Params:  []string{dc.nick, nick, channels},
+			})
+		})
+	case irc.RPL_ENDOFWHOIS:
+		var nick string
+		if err := parseMessageParams(msg, nil, &nick); err != nil {
+			return err
+		}
+
+		uc.forEachDownstream(func(dc *downstreamConn) {
+			nick := dc.marshalNick(uc, nick)
+			dc.SendMessage(&irc.Message{
+				Prefix:  dc.srv.prefix(),
+				Command: irc.RPL_ENDOFWHOIS,
+				Params:  []string{dc.nick, nick, "End of WHOIS list"},
 			})
 		})
 	case "PRIVMSG":