Kline posted a career year in 2004, at 2–2 with a 1.79 ERA. He did not surrender an earned run pitching at home at Busch Stadium, and held left-handed batters to a .143 average. He was also part of the Cardinals' run to the World Series, where they fell to the Boston Red Sox. However, he was not able to pitch in the World Series after aggravating a finger tendon injury in the 2004 NLCS against the Houston Astros.