To determine the predictive value of quantitative motility characteristics produced by the Hamilton Thorn Motility (HTM) Analyzer (Hamilton Thorn Research, Beverley, MA) for in vivo conception. A prospective analysis of 222 couples attending a regional infertility clinic. The measurements were made on a semen sample, and the presence or absence of a treatment-independent conception up to 22 months later was determined. The semen variables were then correlated to conception. University based center for reproductive medicine. The presence or absence of an in vivo conception was recorded in 222 couples in whom the influence on fertility of the female partner was minimized, i.e., normal in terms of history and examination, a regular menstrual cycle, ovulatory (midluteal serum P > 18 nmol/L [5.6 ng/mL]), and the outcome of the hysterosalpingogram was normal. The median follow-up time was 13 months (range, 5 to 22 months). None. Pregnancy. A number of variables were significantly related to time to conception. When a forward stepwise analysis was performed, the total number of spermatozoa was selected on the first step, and average path velocity was selected on the second step. No other variables were selected. The final variables consisted of the total number of spermatozoa and average path velocity. The measurement of quantitative motility and sperm number using a HTM Analyser is of clinical value.