The 1990 American League Championship Series was a best-of-seven series that matched the East Division champion Boston Red Sox against the West Division champion Oakland Athletics. For the second time in three years, the Athletics swept the Red Sox four games to none. The sweep was capped by a Roger Clemens ejection in Game 4 for arguing balls and strikes. The Athletics lost to the Cincinnati Reds in the 1990 World Series.

Oakland won the series, 4–0.

Saturday, October 6, 1990, at Fenway Park in Boston, Massachusetts

The opening game of the series saw a battle of aces, as Oakland sent Dave Stewart to the hill against Boston's Roger Clemens. The game was a scoreless pitchers' duel until the bottom of the fourth, when Red Sox third baseman Wade Boggs launched a solo home run off Stewart. Despite mounting scoring threats in each of the next two innings, the Athletics failed to deliver against the "Rocket."

Oakland finally came through in its half of the seventh, when Larry Andersen replaced Clemens on the mound. Andersen walked Mark McGwire, who then was forced out at second by the next batter, Walt Weiss. A single by pinch-hitter Jamie Quirk moved Weiss to third, and he came home on a sacrifice fly by Rickey Henderson. In the eighth, the A's took the lead via small ball, as José Canseco singled, was sacrificed over to second by Harold Baines, stole third, and then plated on a single by Carney Lansford.

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