Fertilization failure is a serious problem in human in vitro fertilization (IVF) programs and deserves further investigation and management. Two hundred and ninety-four IVF cycles performed at the National Taiwan University Hospital from July 1989 to June 1991 were retrospectively analyzed. Thirty-seven (13%) of the 294 cycles were observed to have fertilization failure. The incidence of fertilization failure in male factor patients was significantly higher (p < 0.05) than in others. Patients with oligoasthenospermia tended to have a higher rate of fertilization failure than patients with oligospermia or asthenospermia alone. In non-male factor patients, a smaller number of oocytes and mature oocytes was found in patients with fertilization failure than in patients achieving fertilization. Sperm penetration assay (SPA) using zona-free hamster eggs was performed for 15 patients with fertilization failure; no correlation was found between SPA and the fertilizing ability of the sperm in vitro. Four patients with severe oligoasthenospermia had repeated fertilization failure in subsequent IVF cycles. The other five patients, including four non-male factor patients and one male factor patient, achieved fertilization in their second IVF trial. Our results suggest that severe oligoasthenospermic patients with repeated fertilization failure should be candidates for micromanipulation of gametes in subsequent IVF trials.