Mary-Kate has entered a clinic for treatment of an eating disorder, sources tell PEOPLE.
"Mary-Kate Olsen recently entered a treatment facility to seek professional help for a health-related issue," Michael Pagnotta, a spokesman for Mary-Kate and Ashley Olsen, who turned 18 on June 13, said in a statement Tuesday. "She is thankful for the encouragement and support of her friends and family who are with her every step of the way."
While Pagnotta did not specify what the health issue was, sources say the treatment is for an eating disorder.
The sisters, who each boast an estimated net worth of $150 million and will enter New York University as freshmen in September, have been the target of public speculation about their weight. As they admitted in a PEOPLE cover story last month: "Being in the public eye, you're labeled that you have an eating disorder," said Ashley. "You have a drug addiction," said Mary-Kate.
Ashley, however, was quick to add: "We don't have problems! There's nothing to worry about."
Earlier this year, the sisters saw the release of their movie New York Minute, and on June 7, the Olsens celebrated their graduation from Campbell Hall, a private school in North Hollywood.