A prospective, controlled study of ovarian function using ovarian ultrasound and daily plasma hormone estimations (estradiol, progesterone [P], follicle-stimulating hormone [FSH], luteinizing hormone [LH]) was carried out on 175 spontaneously cycling patients with unexplained infertility. Forty-one (23.4%) demonstrated luteal phase cyst formation. In 21 cycles the dominant follicle reduced in size after the LH peak (cystic corpus luteum cycles), and in 20 no shrinkage was seen (luteinized unruptured follicles). Progesterone concentrations in the early luteal phase were significantly reduced in the luteinized unruptured follicle cycles. Elevation in plasma FSH was seen in the early follicular and luteal phases of both cyst forming groups and may be due to disturbances in ovarian metabolism. Follicular rupture is important for efficient P release by the corpus luteum.