Failure of in vitro fertilization may occur even though oocyte and semen parameters seem satisfactory. Quantified ultrastructural study of spermatozoa was performed in three cases of failed in vitro fertilization. The results were compared to those of four fertile men. Quantification was achieved by cataloguing cell defects of the spermatozoon heads and mid/principal pieces of the flagella. Using the data from each specimen, the percentages of total cellular abnormalities in the head/mid/principal pieces were established. The percentages of anomalies of the midpiece and of the principal piece were not significantly different between failed cases and controls. The percentage of cell alterations of the head (96-100 vs 75${pm}$3,4%), the percentage of combined anomalies of the head (80-86 vs 52.5${pm}$1.9%), and the percentages of nuclear shape deformation (68-86 vs 47.5${pm}$6.3%), acrosomal defects (86-96 vs 50${pm}$4.3%), and postacrosomal sheath defects (78-88 vs 44.5${pm}$7.2%) of the head were significantly different between failed cases and controls. Due to the cost and time involved in processing semen samples for electron microscopy, the widespread application of this technique to all couples presenting for IVF certainly is not warranted. However, in selected instances electron microscopy may play a crucial role in identifying an occult male factor.