OBJECTIVE To explore the association between semen parameters and spontaneous pregnancy. METHODS Case-control study. METHODS Aberdeen Fertility Centre, Aberdeen, Scotland. METHODS A total of 1,426 untreated couples attending a subfertility clinic with no evidence of azoospermia, anovulation, or tubal disease were observed for 180 weeks. "Cases" were couples who achieved conception spontaneously within the follow-up period; controls were those who did not. METHODS None. METHODS Sperm density, sperm motility, and sperm morphology. RESULTS In women who achieved spontaneous conception (n = 488) the mean (SD) age was 30.6 (4.8) years and median (interquartile range) parity was 1 (0-1), compared with 32.0 (5.4) years and 1 (0-1) in those who did not (n = 938). The median (interquartile range) duration of infertility was 24 (17-36) and 18 (14-24) months in nonpregnant and pregnant couples, respectively. After adjusting for male and female age, parity, year of first visit, and duration of infertility, sperm motility and normal morphology were significantly associated with spontaneous pregnancy, whereas sperm density was not. For motility, the area under the receiver operating characteristic curve was 0.545 (95% confidence interval 0.514-0.577). For morphology, the area under the receiver operating characteristic curve was 0.565 (95% confidence interval 0.534-0.597). CONCLUSIONS In subfertile couples, sperm motility and morphology have limited predictive value for spontaneous conception.