Thirty-one patients whose infertility was attributed to oligospermia were included for treatment by in vitro fertilization and embryo transfer. Three subgroups were defined: severe oligospermia (less than or equal to 5 million motile sperm/ml), moderate oligospermia (6 to less than 12 million motile sperm/ml), and abnormal sperm morphology (greater than 60% atypical). The fertilization rates were compared to those of a normospermic group managed concurrently. A modified overlay technique of sperm preparation is described for oligospermic samples so that the number of motile spermatozoa inseminated into each tube or culture dish containing a mature preovulatory oocyte was similar in each category, within the range 0.5 to 2 X 10(5)/ml. Significantly fewer oocytes were fertilized in the severe oligospermic group (P less than 0.001), suggesting a reduced capacity for fertilization by spermatozoa from severely oligospermic males. The fertilization rate of oocytes was normal in the moderate oligospermic group and those with abnormal morphology, although in the latter there was a significant delay noted in reaching the pronuclear stage (P less than 0.001), and the embryos were at a less advanced stage of cleavage at the time of transfer (0.001 less than P less than 0.01). Pregnancies were achieved in both the severe and the moderate oligospermic groups, with healthy infants delivered from each.