In 1929, a mysterious arm ailment limited him to one game behind the plate and 24 games as a pinch hitter as the Cubs won the National League pennant. Hartnett struck out in all three of his at bats in the 1929 World Series against the Philadelphia Athletics. He rebounded with his best season in 1930, hitting for a .339 batting average with career highs of 122 runs batted in, a .630 slugging percentage and 37 home runs, breaking his own single-season home run record for catchers. Hartnett led all National League catchers in putouts, assists, fielding percentage and in baserunners caught stealing. His single-season home run record for catchers stood for 23 years, until Roy Campanella hit 40 home runs in 1953.