Lee, twice an all-star who belted 37 homers and 116 RBI last season with the Milwaukee Brewers and Texas Rangers, brings badly needed power to an Astros offense that tied the Tampa Bay Devil Rays as worst in the major leagues.
The Astros, however, had to break the bank to acquire Lee.
The $100 million deal is the largest in franchise history, surpassing the five-year $85 million contract extension given to Jeff Bagwell in 2001.
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"This is a historic commitment to winning," Houston general manager Tim Purpura told MLB.com. "When we met this off-season to plot our strategy, we set up our goals for what we wanted to accomplish in the off-season market in the free-agent market and the trade market.
"Our first priority was to increase our offensive production in the outfield. Our second priority was to add a starting pitcher.
"Today, with the signings of Carlos Lee and Woody Williams, we feel like we've addressed those two top priorities in a very significant manner."
Lee, whose contract included a $3 million signing bonus, has a career .286 batting average with 221 home runs and 772 RBIs in eight major league seasons.
The slugging left fielder has also driven in 80 or more runs in each of the last eight seasons and has scored 100 or more four times.
"I'm very excited to be here," said Lee. "I know this is a team where I have a good chance to win a championship. I've always liked this team, they're always very good, and I like this ballpark.
"I like a lot about Houston. That's why I told my agent this was one of the places I'd like to play."
With future Hall of Fame pitcher Roger Clemens and Andy Pettitte yet to decide if they will return to Houston next season the Astros added a little insurance by signing Williams.
A Houston native, Williams posted a record of 12-5 with a 3.65 ERA last season with the National League west champion San Diego Padres.
The 40-year-old right-hander, has a record of 124-101 in 391 career games.
"We're certainly glad to have these two guys are on our side now," said Astros manager Phil Garner. "Carlos will fit very nicely in the middle of that lineup -- oh, my goodness, that's going to be nice."
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