Critical assessment of sperm morphology using specific and stringent criteria is predictive of the subsequent ability of those sperm to fertilize oocytes in in vitro fertilization (IVF). Previous studies have evaluated sperm morphology prior to sperm preparation and, thus, have not assessed the actual sperm used for insemination. We studied the impact of a double wash swim-up technique used for IVF on sperm morphology using the strict criteria of Kruger et al. [5, 6] in 73 consecutive patients undergoing IVF. Pre- and postswim-up morphological assessments were done in a prospective, randomized, and blinded fashion. The mean percentage of normal forms pre- and postswim-up was 19.8% and 23.4%, respectively, an improvement of 18% (p less than 0.05) with 62 of 73 patients showing improvement. Significantly, analysis of the 27 patients with abnormal morphology on initial assessment (normal forms less than or equal to 14%) revealed an improvement in percentage of normal forms from 9.0% to 21.5%, a mean increase of 239% (p less than 0.005). Eighteen of these 27 patients showed improvement in their percentage of normal forms, eight were unchanged, and only one patient had a decline in their percentage of normal forms. We conclude that the double wash swim-up preparation used for IVF substantially improves the percentage of sperm with normal morphology, and that the benefit is most substantial in those cases where the percentage of normal morphology is abnormal.