The growth and rupture of the graafian follicle were studied in 23 women during 40 cycles (20 spontaneous, 20 clomiphene- induced) by the estimation of plasma estradiol levels and ultrasound scan. The mean preovulatory estradiol peak level was 415 pg/ml in spontaneous cycles and 626 pg/ml after clomiphene administration when one follicle was present. Ultrasonic examination revealed the presence of more than one developing follicle in one of 20 spontaneous cycles, and in 11 of 20 cycles after clomiphene treatment. The determination of follicle size and number by ultrasound scan during ovulatidn induction by the routinely used starting dosage of clomiphene has revealed a previously unrecognized incidence of ovarian overstimulation, and may allow the rationalization of ovulatibn induction regimens.