The outcome of in-vitro fertilization and embryo transfer (IVF-ET) was compared in 76 patients with polycystic ovaries (PCO) diagnosed on pre-treatment ultrasound scan, and 76 control patients who had normal ovaries and were matched for age, cause of infertility and stimulation regimen. Despite receiving significantly less human menopausal gonadotrophin (HMG), patients with PCO, as compared with controls, had significantly higher serum oestradiol levels on the day of human chorionic gonadotrophin administration (5940 +/- 255 versus 4370 +/- 240 pmol/l, P < 0.001), developed more follicles (14.9 +/- 0.7 versus 9.8 +/- 0.6, P < 0.001) and produced more oocytes (9.3 +/- 0.6 versus 6.8 +/- 0.5, P = 0.003). However, fertilization rates were reduced in the PCO patients (52.8 +/- 3.4% versus 66.1 +/- 3.4%, P = 0.007). There was no significant difference in cleavage rates. The pregnancy rate/embryo transfer was 25.4% in the PCO group and 23.0% in the group with normal ovaries. There were three high order multiple pregnancies in the PCO group compared with none in the group with normal ovaries. Of the PCO patients, 10.5% developed moderate/severe ovarian hyperstimulation syndrome (OHSS) compared with none of the controls (P = 0.006). Patients with and without PCO undergoing IVF have comparable pregnancy and livebirth rates. However, it is important to diagnose PCO before ovarian stimulation is initiated as these patients are more likely to develop moderate or severe OHSS following IVF-ET.