A total of 31 clomiphene citrate/human menopausal gonadotrophin (HMG)/human chorionic gonadotrophin (HCG)-stimulated cycles in 28 patients were investigated to determine the fate of each of the matured follicles. A standard stimulation regimen was adhered to, and ultrasound as well as hormonal monitoring was performed. All follicles were measured by vaginal ultrasound at -12, +35 and +45 h relative to HCG administration and at 7 days after HCG administration. Of the 220 follicles, 107 (48.6%) ruptured. The number of ruptured follicles per cycle was correlated with the mid-luteal progesterone concentration (r = 0.63, P = 0.0005). The probability of follicular rupture was related to follicular diameter at 12 h before HCG administration; 6% of follicles < 12 mm in diameter ruptured compared with 87% of follicles 18-19 mm. A complete luteinized unruptured follicle (LUF) syndrome was observed in six cycles (20%). In these cycles, follicular growth and oestradiol, progesterone, luteinizing hormone (LH) and follicle stimulating hormone (FSH) concentrations at 12 h before HCG administration were similar to those in cycles with follicular rupture. However, mid-luteal progesterone concentrations were lower in complete LUF cycles (46.97 +/- 8.95 nmol/l versus 108.74 +/- 12.27 nmol/l; P = 0.02). These data demonstrate that in stimulated cycles many follicles, usually the smaller ones, fail to rupture, even after HCG administration. Complete LUF syndrome, despite a strong exogenous ovulatory signal, and the absence of any difference in peri-ovulatory hormonal parameters, indicates that the defect causing LUF resides in the follicle itself and/or hormonal changes during the follicular phase.