Mifepristone (RU486) is a potent antiprogestagen, and at high doses it also acts as an antiglucocorticoid drug. Mifepristone, administered as a single 600 mg dose, is commonly employed to induce medical abortion in conjunction with prostaglandins. The long-term safety profile of mifepristone, especially at high doses, is less well-established. Long-term mifepristone is considered efficacious in treating uterine myomas, endometriosis (25--100 mg/day), and possibly in inoperable meningiomas (200 mg/day), as well as inoperable Cushing's syndrome. Many animal studies document an antiproliferative effect (antioestrogenic), as do some reports in humans. However, there are also data to suggest that, as an antiprogestagen, mifepristone may promote an unopposed oestrogen milieu, and thus have a proliferative effect upon the endometrium. We hereby describe the first reported case of an adolescent female with Cushingoid features and morbid osteoporosis who was treated with mifepristone for its antiglucocorticoid effect (400 mg/day) in an attempt to prevent further bone loss. The patient's striae, weight gain, and buffalo hump markedly improved, and further bone loss was halted. However, with each of the two 6-month courses of mifepristone (9 months apart) she developed massive simple endometrial hyperplasia and a markedly enlarged uterus. This reversed to normal after cessation of mifepristone treatment. In conclusion, High doses of the antiprogestagen mifepristone over a prolonged period of time may promote an unopposed oestrogen milieu leading to endometrial hyperplasia. Therefore, interval pelvic imaging in women who receive long-term mifepristone may be prudent.