After one more season in Finland, Lehtonen came to North America in 2003. He spent most of 2003-04 with Chicago of the American Hockey League but went 4-0-0 in a late-season callup, allowing five goals in four Thrashers wins.

Lehtonen became the highest-drafted Europe-born goalie in NHL history when the Atlanta Thrashers selected him No. 2 in the 2003 NHL Draft. That occurred after Lehtonen began the 2001-02 season as the backup with Jokerit in Finland's top pro league, but took the starting job, was named the league's top goalie and its playoff MVP after sparking his team to the championship.

Lehtonen became the highest-drafted Europe-born goalie in NHL history when the Atlanta Thrashers selected him No. 2 in the 2003 NHL Draft. That occurred after Lehtonen began the 2001-02 season as the backup with Jokerit in Finland's top pro league, but took the starting job, was named the league's top goalie and its playoff MVP after sparking his team to the championship.

After one more season in Finland, Lehtonen came to North America in 2003. He spent most of 2003-04 with Chicago of the American Hockey League but went 4-0-0 in a late-season callup, allowing five goals in four Thrashers wins.

He went 20-15-0 as a rookie in 2005-06, then won 34 games in 2006-07, when the Atlanta won the Southeast Division and qualified for the Stanley Cup Playoffs for the first time since entering the NHL in 1999. However, Lehtonen missed 16 games early in the 2007-08 season with a groin injury and dropped to 17 wins, then won 19 games in 2008-09. The Thrashers missed the playoffs each time.

Atlanta traded Lehtonen to the Dallas Stars on Feb. 9, 2010. He quickly became a fixture in Dallas, winning at least 31 games in each of the next four full seasons and leading the NHL in games played (65) and minutes played (3,804) in 2013-14.

Lehtonen remained with the Stars through 2017-18, earning his 300th NHL victory when he defeated the New York Islanders on Dec. 13, 2017. He finished 310-233-67 with a 2.71 goals-against average, .912 save percentage and 38 shutouts in 649 League games, and was 8-9 with 3.30 GAA, .887 save percentage and two shutouts in 19 playoff games.

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