A quantitative ultrastructural study of sperm was performed in 72 cases of male infertility. Results were compared to those of 6 fertile men. In 28 cases semen parameters were distrubed. Thus, the poor quality of these sperms is confirmed by electron microscopy which carries out informative data on flagellar defects in 8 cases of out 12 in severe impairment of motility. In 44 cases, unexplained failure of in vitro fertilization or very low cleavage rate were observed even though oocyte and semen parameters seemed satisfactory. There was no significant difference between the anomaly rates for the flagellum components, in patients and controls. Conversely the percent of cell alterations and acrosomal defects (15 cases) or combined defects of the head (19 cases) were significantly higher in 34 out of 44 patients. Therefore in such cases electron microscopy with quantified analysis can supply valuable evidence about poor quality of sperm not visualized by light microscopy and can lead to a better predictive information on the fertilizing ability of these sperms.