In vitro fertilization was attempted in 25 consecutive cycles as a treatment for male factor infertility. These cycles were compared with 25 control cycles in which a male factor was not present. The fertilization rates for mature oocytes in male factor cases and controls were 17% and 70%, respectively (P less than 0.001). For immature oocytes, the fertilization rates for study patients and controls, respectively, were 16% and 57% (P less than 0.001). Embryos were available for transfer in 36% of study cycles and in 88% of control cycles (P less than 0.01). The pregnancy per transfer rate for study cycles was 33% and for control cycles was 32%. The hamster ova penetration assay was 100% specific and 50% sensitive in predicting fertilization in vitro among male factor patients.