Harden began the 2004 season with the Sacramento River Cats, coming out of spring training as the A's 5th starter. But the Athletics had two off days in the first eight days of the season, and they optioned him down to get a start in the minor leagues. He pitched in one game, losing 5–3 to the Edmonton Trappers. The Athletics called him up on April 10, and he put together a very solid season with an 11–7 record and an ERA of 3.99. Harden compiled an 8–2 record and an ERA of 3.49 after the All-Star break. He ranked 8th in the AL with 167 strikeouts, and was tied for 7th with 81 walks. Harden allowed just 16 home runs in 189.2 innings, an average of 0.76 per nine innings, which was tied for 4th lowest in the American League. He threw the fastest fastball of all major league starters, averaging 94.3 miles per hour.