The results of subzonal sperm insertion (SUZI) have been retrospectively analysed in a subset of patients with normal sperm counts who were found to require SUZI because of poor or absent fertilization of zona-intact oocytes. This patient group is of particular interest because male factor-related infertility cannot be due to insufficient numbers of spermatozoa reaching the oocytes. Thus, failed fertilization can be attributed to deficiencies in one or more steps in the fertilization process, and SUZI provides a method of distinguishing defects of zona pellucida penetration from gamete fusion. A total of 26 such patients were treated identically to and concurrently with a much larger group of SUZI candidates who typically suffered from oligozoospermia, and fertilization results were compared. Fertilization rates after SUZI were higher in patients with normal counts than in oligozoospermic patients (51 and 26% respectively), indicating that the proportion of spermatozoa capable of fusing with the oocyte is the same or higher in the group with normal counts. In addition, nearly all SUZI procedures led to fertilization (23/26), with two out of three failed fertilizations occurring in cases where two or less oocytes were manipulated, results which further indicate that failed fertilization in these patients is not due to a defect at the level of gamete fusion. These findings suggest that infertility in these patients is based upon the inability of the spermatozoa to reach the oolemma and thus, that their fertility defect resides at the step of zona penetration.