The Phillies also received veteran infielder Eric Bruntlett in exchange for outfielder Michael Bourn, reliever Geoff Geary and Minor League third baseman Mike Costanzo.
Lidge, 30, spent the bulk of the past four seasons as Houston's closer, and he will step into the same role with Philadelphia, pushing Brett Myers back into the starting rotation.
"He's got outstanding stuff and has been a guy who has pitched very well since he first came up," Phils GM Pat Gillick said. "It's no secret we've been looking for pitching, and this certainly helps us there. Bruntlett's a guy who can play several positions, pinch-hit, run and help out in a lot of ways."
Lidge finished 5-3 with 19 saves and a 3.36 ERA this past season, though his season was somewhat spotted with eight blown saves. He had surgery after the season to repair torn cartilage in his right knee, and he missed nearly a month of action with a strained left oblique. For his career, Lidge owns 123 saves and a 3.30 ERA.
As recently as two seasons ago, Lidge ranked among the top handful of relievers in the Major Leagues, finishing in the top 10 of National League Cy Young Award voting in 2004 and making his first All-Star team the following year. But ineffectiveness has undermined him over the past two seasons, and he temporarily lost his closer job on multiple occasions since.
The relocation to Philadelphia will plunk Lidge back in the spotlight, after he missed out on the playoffs for two straight years in Houston.
"I guess it'll be like when we were in the middle of the postseason, with all of the media and attention we get," Lidge said. "That kind of adrenaline and excitement will hopefully be beneficial to me. When you have a lot of adrenaline in situations, it might push you and you'll excel. I'm optimistic about it."
Bruntlett, 29, hit .246 in 80 games with the Astros last season, mostly as a reserve shortstop. He also spent time at third base and every outfield position, and he is a career .250 hitter. Bruntlett should supplant Abraham Nunez as the team's utility infielder.
"Brad Lidge and Eric Bruntlett have been outstanding members of the Astros organization," Houston general manager Ed Wade said in a statement. "We weren't anxious to trade either player, but this deal makes sense for both the Astros and the Phillies."
To acquire Lidge, the Phillies traded away an important defensive piece in Bourn. Best known for his highlight-reel catches as a defensive replacement and spot starter, Bourn, 24, hit .277 in his first full season as a Phillies reserve. He stole 18 bases and was caught just once. A Houston native, the trade will allow him to return to his hometown as a full-time starter.
Bourn's absence will put greater pressure on the Phillies to re-sign center fielder Aaron Rowand, as Bourn could have been a candidate to replace him in at least a platoon situation. But with Greg Dobbs likely to spend the bulk of his time at third base, the absence of both Bourn and Rowand would force the Phillies to give a starting job to reserve outfielder Jayson Werth.
The Phillies also gave up Geary, 31, who regressed a bit after a career year in 2006. Geary bounced between Triple-A and the Majors in '07, finishing with a 3-2 record and 4.41 ERA.
The last piece of the trade, Costanzo, was once considered one of the organization's top prospects before a down year in 2006. The Phillies' second-round choice in the 2005 First-Year Player Draft, he hit 27 homers this past season, spending its entirety with Double-A Reading. Costanzo, 24, is a Pennsylvania native and lifelong Phillies fan.
While Lidge is the marquee name in the trade, the fallout centers even more around Myers. Gillick told the Philadelphia Inquirer on Wednesday night that the acquisition would free Myers to return to the starting rotation, where he flourished for the first five years of his career.
There was no letdown after manager Charlie Manuel moved Myers to the bullpen in April, as he finished with 21 saves in 24 opportunities. Myers stated publicly on several occasions that he preferred closing to starting, though he affirmed on Wednesday he was open to either role.
"Pat asked me how I felt about it, and I said I'd do whatever was best for the team," Myers told the Inquirer. "It's a little disappointing because I felt closing was good for me. But the team has other needs, and I can do both. I'm not surprised. You could read between the lines."
Myers held a 2.87 ERA as a reliever this past season, one of the few bright spots for a bullpen that ranked 24th in the Majors with a 4.41 ERA.
Anthony DiComo is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.