McGwire, Mark David (məgwĪrˈ) [key], 1963–, American baseball player, b. Pomona, Calif. A muscular first baseman who was a college and Olympic (1984) star, McGwire broke into the American League as Rookie of the Year in 1987 with the Oakland Athletics, hitting a record 49 home runs for a rookie season. In succeeding years he sometimes struggled for hitting consistency but showed great power, hitting 52 homers in 1996. In 1997 he was traded in midseason to the St. Louis Cardinals of the National League, and he completed his second successive year with 50 or more home runs, a feat achieved previously only by Babe Ruth. In 1998 he and Sammy Sosa of the Chicago Cubs mounted an assault on the single-season home-run record (61) held by Roger Maris. McGwire hit two in the year's final game to end with 70; Sosa finished with 66. In 1999 the pair repeated their achievement, McGwire hitting 65 homers, Sosa 63. McGwire's single-season home-run record was broken in 2001 by Barry Bonds. Plagued by injuries, McGwire retired the same year with 583 lifetime home runs. His achievements have been tarnished by the fact that they were accomplished with the aid of anabolic steroids (which was long suspected and publicly admitted in 2010). McGwire became the Cardinals hitting instructor in 2010.