Twelve normally cycling women received during the postovulatory phase 100 mg oxymetholone daily for 7 days or until menstruation occurred. As compared to control cycles (mean duration 29.6 days) oxymetholone shortened the average cycle length by 6.0 days (p = 0.00003). Similar effects were achieved with a daily dose of 25 mg, but not with a single dose of 250 mg. In 6 women treated with 100 mg daily, blood levels of LH, progesterone, estradiol and estrone were estimated during control and treatment cycles. The luteal phase dated from LH peak was shortened by a mean of 3.5 days (p = 0.017). This effect was not evident unless treatment started shortly after ovulation. Oxymetholone treatment was associated with a premature decrease of progesterone blood levels. Estradiol and estrone blood levels were not substantially influenced by the treatment. Urinary 17-hydroxycorticoid and 17-ketosteroid excretion remained within normal limits.