Williams, Aardsma was traded to the Chicago Cubs for veteran pitcher LaTroy Hawkins on May 28, 2005. He spent the season in the minor leagues before returning to the big leagues with the Cubs in 2006, posting a 3–0 record and 4.08 ERA in 45 relief appearances, finishing nine games. Aardsma was especially effective against left-handed hitters, holding them to a .190 (12-for-63) batting average against. Chicago White Sox After a solid 2006 season with the Cubs, Aardsma, along with minor leaguer Carlos Vásquez, was sent across town to the Chicago White Sox in exchange for reliever Neal Cotts. Aardsma started the 2007 season strong. In April, he posted a 1.72 ERA while recording 23 strikeouts in only 15.2 innings pitched; he struck out at least one batter in each of his first 13 appearances of the season. On April 4, Aardsma matched a career high with five strikeouts against the Cleveland Indians. On April 11, as the White Sox visited the Oakland Athletics, he recorded his first American League win. In May, however, troubles mounted and Aardsma finished the month with a season ERA to that point of 4.73 and an ERA of 9.00 for the month. Following June 2, Aardsma was optioned to Triple-A Charlotte. He was recalled on June 19, but continued to struggle in his last appearances with the team. Overall, he ended with a 6.40 ERA in 25 games. Boston Red Sox On January 28, 2008, the Boston Red Sox acquired Aardsma from the White Sox for pitching prospects Willy Mota and Miguel Socolovich. He pitched in 47 games for the team that year, going 4–2 with a 5.55 ERA. He also spent two games in the minors, posting a 0.00 ERA for the Triple-A Pawtucket Red Sox. , Aardsma appears first alphabetically on the all-time list of Major League players. Seattle Mariners Less than a year after joining the Red Sox, Aardsma was traded to the Seattle Mariners on January 20, 2009, for minor league pitcher Fabian Williamson. On April 10, he recorded the first save of his career, pitching 2 innings of relief against the Oakland Athletics. The Mariners gave Aardsma a chance to close a game behind Brandon Morrow. Afterward, he became the team's official closer. Aardsma was a projected pick for the All-Star Game, but failed to make both the starting and the reserve roster. He followed an outstanding 2009 season