Birmingham City won for the first time in seven Championship matches to dent Middlesbrough's play-off ambitions.
Scott Hogan's first-half goal was enough to secure all three points at the Riverside Stadium, with the striker finishing off a slick team move by tapping in Maxime Colin's low cross.
Boro almost equalised before half-time when Sam Morsy's header bounced down off the crossbar and on to the goalline before Blues keeper Neil Etheridge smothered the ball.
Duncan Watmore had the best chance to level the game in an improved second-half showing by the hosts, but saw his close-range half-volley saved by Etheridge.