Ashley will not attend the Australian premiere of "New York Minute" - the twins' new film - in Melbourne next week, said Robert Thorne, the chief executive of Dualstar Entertainment Group, their entertainment empire, in a statement.
"Ashley Olsen has decided to be with her family at this time," Thorne said.
Earlier this week, U.S. magazines reported that Mary-Kate was suffering from anorexia nervosa. Anorexics are obsessed with losing weight and often exercise excessively or refuse to eat.
Mary-Kate entered "a treatment facility to seek professional help for a health-related issue," her publicist Michael Pagnotta said Tuesday, but did not elaborate.
"While it is unfortunate that plans have been canceled, Mary-Kate and Ashley appreciate everyone's support and understanding at this time. They look forward to coming to Australia and New Zealand in the very near future," Thorne's statement said.
Mary-Kate and Ashley turned 18 on June 13, assuming control of Dualstar Entertainment. They are estimated to be worth US$150 million each.
They've been acting together since they were 9 months old, most notably on the sitcom "Full House" and in a series of direct-to-video movies. They have also attached their names to a line of merchandise including clothing, beauty products and home decorating materials.
"New York Minute" did poorly at the U.S. box office after its May release.