Claudio Yacob's 42nd-minute header has given West Bromwich Albion a half-time lead over in-form Arsenal at The Hawthorns.
Early on the Gunners had a couple of half-chances before they almost fell behind to a slice of bad luck in the 14th minute when Saido Berahino's long-range strike took a wicked deflection but did not beat Wojciech Szczesny, who got down low to tip it away.
Ten minutes later Jack Wilshere threatened with a shot that went wide after some neat passing on the edge of the Baggies' area from the visitors.
Aaron Ramsey then tested Boaz Myhill with a shot through a crowd of players before West Brom broke the deadlock three minutes before the interval.
Arsenal failed to clear the lines from a corner and Morgan Amalfitano swung the ball back into the middle where Yacob bulleted a header past Szczesny.
