Hills Star Admits to Bulimia, Triggered by "Skinny" Costars

Hills star Stephanie Pratt opens up for the first time in the new issue of Us Weekly about hating her body and her terrifying secret struggle with bulimia -- sparked, she says, by the pressure of appearing alongside her super-thin friends on the hit MTV reality show.

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The 5-foot-7 23-year-old reveals she began bingeing and purging after watching a scene she shot with Lauren Conrad in 2007 after first joining the cast: "I was horrified. I remember saying, 'I can't believe how huge I look walking over to Lauren.'"

See what Hills stars looked like as babies.

Another time, when filming in Hawaii in bikinis, the size 0-2 Pratt - in a bikini and cover-up -- asked costar Audrina Patridge, "What looks better, 'shirt on or off?'" Patridge said 'On.'" Says Pratt, "The producers never put pressure on us to lose weight," but adds, "It's embarrassing working with skinny girls."

See photos of scary-skinny stars.

The new issue of Us Weekly hits stands today.