Mischa Barton doesn't let a little vomit ruin a good time.

This much was evident Thursday night when the 21-year-old actress hit the NYC hotspot The Anchor at 10:30 p.m. with pals Jamie-Lynn Sigler, 25, and Kirsten Dunst, 24, and celebrated to the point that she made herself sick.

"They were partying, going crazy," an onlooker tells Usmagazine.com. "At one point Mischa suddenly ran out the door and threw up outside. Jamie-Lynn ran outside after her, but it wasn't a big deal. They were laughing."

The witness added: "They just went back inside and continued to party."

The trio left after 3 a.m.