By Michelle Tauber Jill Smolowe
July 05, 2004 12:00 PM

Like nearly everything they do, Mary-Kate and Ashley Olsen had planned to spend their 18th birthday on June 13 side by side. But when the much-hyped day arrived, Ashley was left to celebrate without her twin sister, first poolside at the Beverly Hills Hotel and then with a jaunt to Mexico with two pals. One week later the reason for Mary-Kate’s absence became clear: The star, recently the focus of speculation regarding her increasingly frail-looking body, had been admitted to an undisclosed facility for treatment related to an eating disorder. “This is a challenge that Mary-Kate has made a decision to face,” says her rep Michael Pagnotta. “This is a challenge she will meet.”

The admission struck a sad and troubled note for Mary-Kate, who along with Ashley has turned their image as positive, clean-scrubbed role models into a business empire that grossed more than $1 billion in sales last year. Yet to friends, family and even casual onlookers, the subject of Mary-Kate’s health has been a source of concern in recent months. Although the petite twins (Mary-Kate is 5’2″; Ashley is 5’1″) have always been slender, Mary-Kate appeared to be painfully thin of late. So widespread was the talk about her weight that the actress herself poked fun at it when the twins hosted the May 15 Saturday Night Live; playing paparazzi photographers, they shouted, “Mary-Kate, you’re so skinny—eat a sandwich!” Sadly, beneath the playfulness was a health crisis. “There was an intervention,” says someone who has known the Olsens for several years, adding that the twins’ father, Dave, and Mary-Kate’s therapist committed her to a treatment facility soon after her June 7 high school graduation. “They finally reached the point where they had to act. They didn’t want to find her dead on the floor from not eating.”

Those close to the star say that there was no single incident that triggered the decision to seek professional treatment; rather, Mary-Kate’s problems had steadily worsened with time. One source says that the change in Mary-Kate, who dates Boston University student David Katzenberg, 21 (his dad is DreamWorks studio cofounder Jeffrey Katzenberg), first became evident two years ago. “She looked different,” says the source. “She developed circles under her eyes.” Staging the intervention before Mary-Kate turned 18 meant that her parents still had a legal right to seek treatment for her.

It isn’t the first time her family has tried to aid Mary-Kate, whom close pals call MK. According to one source, after the actress got into a minor car accident about a year and a half ago her father tried motivating her to eat by telling her she couldn’t get her black Range Rover back until she gained some weight. Another source says the family hired someone to monitor Mary-Kate’s eating. “Sometimes she was taken out of school to eat,” says a former classmate at Campbell Hall high school. “During the last year she had an adult eating with her most of the time. Everyone knew how skinny she was.”

And yet not everyone recognized it as a problem. “I didn’t see the signs of [an eating disorder],” says director Dennie Gordon, who helmed the twins’ spring comedy New York Minute. “I had a lot of meals with her, and it didn’t seem there was anything wrong.” Adds Dr. Drew Pinsky, the TV psychologist turned actor who played the twins’ father in New York Minute: “She was the sweetest of the two. But I didn’t notice anything. She; hid it well.”

Such deception is typical behavior among eating-disorder sufferers, many of whom go to elaborate lengths to disguise their problems. “Everyone was very angry with Mary-Kate at first,” says a source. “She’s lied so much and for so long. It’s part of the illness.” In fact, both Mary-Kate and Ashley flatly denied such problems in an interview with PEOPLE in April. “Being in the public eye, you’re labeled that you have an eating disorder,” Ashley said at the time. “You have a drug addiction,” said Mary-Kate. “We don’t have problems!” declared Ashley.

The fraternal twins—MK has long been known as the free-spirited one; Ashley as the sophisticated one—have also put on a united front dealing with other recent challenges, including the dismal $13.7 million box office for New York Minute. “We were really disappointed,” says Gordon. “They were incredibly mature about it. They said, ‘Okay, I guess we can’t open a movie just yet. Next!’ ”

Showbiz vets since making their debut on Full House at just 9 months old, the girls have long been praised for their business savvy and solid grounding. But others have wondered if the stress placed on their slender shoulders would eventually take a toll, as it has on other young Hollywood stars (see sidebar). “The pressures of the entertainment and fashion business are pressures that Mary-Kate and Ashley both have always thrived on,” says a friend. That said, “there’s no doubt that the pressure of being successful, running a business and planning for college—that’s a lot.” Still, adds the friend, “I don’t think you can draw a straight line between the pressure in anybody’s life to this sort of illness.” Says Carolyn Costin, the director of the Eating Disorder Center of California and the Monte Nido Treatment Center in Malibu: “We look at it like there’s a gun, a bullet and the trigger: The gun is biological predisposition, the culture is the bullet and something like the stress of being a celebrity is what pulls the trigger.”

Not helping matters is the ever-present skinny sweepstakes among young women, many of whom worship stars like Mary-Kate as a source of “thinspiration,” Shortly after Mary-Kate’s appearance at a May premiere, Web surfers on a bulletin board at posted messages like, “I’m convinced she’s ana [a slang term for anorexic]!” Another online follower took notice of Mary-Kate’s red string bracelet, which some teens and young adults wear as a signal of their “ana” pride and to remind themselves not to eat. (Her rep’s response: “I don’t believe that’s true [of Mary-Kate].”)

What is certain is that Mary-Kate must now do the difficult work of recovering (see box). She has canceled her plans to promote New York Minute in Australia and New Zealand (Ashley plans to go alone this week), but she remains committed to attending New York University with her sister in the fall. “The focus right now is for Mary-Kate to get well,” says a friend. “She’s taking care of her health, and there’s a certain peace that comes from that.”

Michelle Tauber and Jill Smolowe. Tom Cunneff and Ulrica Wihlborg in Los Angeles and Allison Adato, Natasha Stoynoff, Mark Dagostino and KC Baker in New York