The relationship between computer based measures of sperm morphology and motility, and male infertility was examined. The study group consisted of 569 males, mean age 38 years (yr), who were subjects in two large studies investigating the health effects of military service in Vietnam. Semen samples were collected and analyzed by a computer assisted sperm analysis instrument to measure spermatocyte area, perimeter, length/width ratio, and roundness as indicators of morphology and percentage motile cells, percentage progressive cells, mean linear velocity, mean straight line velocity, mean amplitude of lateral head displacement, and mean beat or cross frequency as measures of spermatocyte motility. Associations between the spermatocyte morphology and motility parameters and infertility, defined as the inability to father a child after 1yr of trying and small number of children fathered, were examined by logistic regression techniques after adjusting for possible confounding by race, education, current alcohol use, smoking, and illegal drug use. Approximately 1% of the subjects had a history of varicocele or epididymitis, conditions associated with infertility. About 40% had fathered a child during the past 5yr. Among the morphological parameters, an increase in spermatocyte length/width ratio above the ninety fifth percentile (that is, a higher proportion of elongated sperm) was significantly associated with both measures of infertility after adjusting for potential confounders. All measures of spermatocyte motility were negatively associated with infertility. After adjusting for the confounders, only the percentage of progressive cells was associated with infertility and only with one measure, difficulty in fathering a child. The authors conclude that spermatocyte length/width ratio and percentage of progressive spermatocytes can be related to male infertility. The spermatocyte length/width ratio appears to be the more important marker for male infertility.