Cubs on January 21, 2004. After rehabbing from his surgery, he joined the Cubs on August 1 and appeared in 23 games, all out of the bullpen. He picked up his first MLB career save on September 15, 2004 against the Pittsburgh Pirates. In , after starting 6 games, Dempster was named the closer for the Cubs, replacing LaTroy Hawkins. He collected 33 saves in 35 opportunities, the best save percentage in the league; in both blown saves, he collected the win. He is the only Cub and one of just three players ever to both start a game and collect 30 saves in the same season. On October 1, 2005, the Cubs signed him through 2008 in a US$15.5 million contract extension. He emerged as one of the team's most consistent relievers in . At one point, Lou Piniella, the Cubs' manager, was considering promoting Dempster to a starting pitcher. However, the idea was short lived, as a struggling bullpen forced Dempster to remain as closer. He recorded 16 saves in 18 opportunities before missing two weeks of playtime on account of an injured oblique muscle. During 2008 spring training, Dempster's performance resulted in his being designated to the third spot on the starting rotation, with the closer job going to Kerry Wood. On April 3, 2008, he earned his first win as a home starter since a 2002 win with the Marlins against the Cubs. On May 15, 2008, Dempster recorded a career high 12 strikeouts in a game. He pitched innings, and gave up only 6 hits. On July 8, 2008 he became the first Cubs pitcher in 31 seasons to start 10–0 at home. Dempster finished the season with 17–6 record, with 2.96 ERA and 183 strikeouts in innings. Dempster threw sliders 32.9% of the time in 2008, more than any other NL starting pitcher. Dempster opened the Cubs 2008 postseason campaign against the Los Angeles Dodgers on September 26, 2008. Dempster accumulated a high pitch count early and, after he was unable to get outs with 0–2 counts twice, he surrendered a grand slam to James Loney and was pulled in favor of Sean Marshall. In 2008, Dempster led the majors in sacrifice hits, with 19. Following the 2008 season, Dempster re-signed with the Cubs for four years and a reported $52 million. In May 2009 MLB indicated that the Commissioner's Office would review Dempster's