Barbara Bush's purse was snatched at a Buenos Aires restaurant during a night on the town, but the Secret Service agents did not notice, ABC said, citing police sources.
Kim Bruce, spokeswoman for the Secret Service, which provides protection for the president and family, said: "We don't have any comment on that."
The White House did not immediately return telephone calls seeking comment.
Barbara, named for the president's mother, and her twin sister, Jenna, turn 25 Saturday.
The twins were dining in San Telmo, Buenos Aires's picturesque arts and dining neighbourhood.
ABC reported that the theft was not the only difficulty the Secret Service faced during the fraternal twins' two-week visit.
One of the agents got into an "altercation" after a night on the town and was beaten, the television news organisation said, citing police reports.

Copyright AFP (Agence France-Presse), 2006