Freezing and storage of human male gametes is associated with a reduction in the overall semen quality and establishment of pregnancy. This study was done to evaluate the integrity of sperm head ultrastructure (SHU) with computerized and vapor freezing. Comparisons were made between the effect of cryopreservatives glycerol (G) and dimethylsulfoxide (DMSO) on SHU. Twelve ejaculates from five proven fertile donors were studied with the use of routine semen analysis, zona-free hamster ova, and SHU. Both cooling processes, regardless of the preservative used, significantly reduced sperm function and the number of SH with intact plasma membranes. The staged cooling technique was substantially superior to vapor freezing in all parameters analyzed (P less than 0.01). G was less detrimental to the postthaw SHU than 1 M DMSO. A significant positive correlation (r = 0.98; P less than 0.01) was noted between the total number of intact SH and motile sperm. Computerized freezing in a G-diluted semen rendered a sperm environment that allowed the highest number of forms with intact SH membranes and with the best chances to penetrate zona-free hamster ova.