In the study period (June 1984 to December 1986), 2232 cycles were stimulated in 1294 patients. Ultrasound-directed oocyte recovery (UDOR) was performed as an ambulatory procedure in 1737 (77.8%) cycles, resulting in 1375 embryos transfers (ET). Age, etiology, menstrual cycle length, number of oocytes collected, and number of embryos transferred were important determinants of the outcome. The number of attempts at in vitro fertilization did not affect the clinical pregnancy rate (CPR). In patients receiving four embryos, the CPR appeared to be highest when up to seven embryos were available for transfer. The fertilization rate in an individual cycle had a good prognostic value, the implantation rate being highest when 7 to 9 oocytes were retrieved and greater than 60% of these were fertilized. When 10 or more oocytes were collected, the implantation rate showed a progressive decline, regardless of the fertilization rate. Furthermore, multiple pregnancies failed to occur when greater than 12 oocytes were retrieved or more than eight embryos were available for transfer. These data suggest that, in excessively stimulated cycles, the quality of oocytes and embryos or uterine receptiveness may be suboptimal, and the transfer of more than four embryos is unlikely to increase the success rate.