Caldwell leaped from Class A to the Padres in 1971, his first pro year. Swapped to the Giants for Willie McCovey and Bernie Williams, he blossomed with a 14-5 mark in 1974. He then underwent elbow surgery and had trouble regaining his form. The property of the Cardinals and Reds in 1977, he finally landed in Milwaukee. In 1978 he was AL Comeback Player of the Year, going 22-9 (2.36) with a league-high 23 complete games and shutting out the World Champion Yankees three times. In 1979 his .727 winning percentage (16-6) was the AL best; his eight consecutive victories set a Brewers record. After helping Milwaukee to the 1982 pennant with a 17-13 mark, he was pounded by the Angels in his LCS start, but shut out the Cardinals in the World Series opener and also won Game Five.
Caldwell used a three-quarters delivery, but threw sidearm to some lefties. His sinker was most effective, though he was accused of throwing a spitter. By 1984 his arm wore out, and he lost his fastball. Iron Mike retired as the Brewers’ leader in complete games.