Serial ultrasonic scans of follicular development were performed throughout 46 spontaneous cycles in 20 healthy female volunteers. Human chorionic gonadotropin was then given to induce follicle rupture on a particular day. Luteinized unruptured follicles (LUFs) were seen in 10.7% of untreated cycles. When prostaglandin synthetase inhibitor drugs were administered over the ovulatory period the incidence of LUF was greatly increased (to 50% with azapropazone and 100% with indomethacin). Serum estradiol concentrations and length of luteal phase were unaltered in LUF cycles. Progesterone concentrations were lower in the first half of the luteal phase if follicles remained unruptured.