"Iron Man" vanquished two new U.S. box-office rivals to finish at No. 1 for a second straight week with an estimated $50.5 million, Box Office Mojo said Sunday.

The Marvel Comics-Paramount Pictures release -- starring Robert Downey Jr. in the title role -- has grossed an estimated $177.3 million in its first two weekends in release, the box-office tracker reported.

Two major new releases -- "Speed Racer" and "What Happens in Vegas" -- battled it out for second place.

"Speed Racer," which got mostly poor reviews, grossed an estimated $20.2 million. "What Happens in Vegas," a romantic comedy starring Cameron Diaz and Ashton Kutcher, took in an estimated $20 million to debut at No. 3, Box Office Mojo said.

Two other romantic comedies rounded out the top five -- with "Made of Honor" taking in $7.6 million and "Baby Mama" adding $5.7 million to its take.

"Forgetting Sarah Marshall" was No. 6 with $3.7 million, followed by "Harold and Kumar Escape from Guantanamo Bay" with $3.5 million; "The Forbidden Kingdom," $1.9 million; "Nim's Island," $1.3 million; and "Redbelt," $1.1 million. (c) UPI