Estrogen (E)/progestin therapy for functional ovarian cysts is widely used in clinical practice, but the efficacy of this treatment has not been determined in controlled trials. In this study, we examined the effect of E/progestin administration in a group of infertility patients enrolled in a program of ovulation induction who had cysts identified by transvaginal sonography. Patients were randomized to receive either norethindrone 1 mg/mestranol 0.05 mg/d (group A, n = 24) or no treatment (group B, n = 24) for up to 6 weeks. Patients were re-evaluated by sonography at 3, 6, and 9 weeks after entry into the protocol. The ages, mean cyst diameters, and proportions of patients having received gonadotropins in the previous menstrual cycle were not significantly different among the two groups. All patients who had a sonographic abnormality persisting for 9 weeks were surgically explored and found to have pathological cysts. The rate of disappearance of functional ovarian cysts was not affected by E/progestin treatment.