To explore prenatal risk factors that are common to testicular cancer and cryptorchidism, two parallel case-control studies were conducted in Denmark. Information about characteristics of the mother, the pregnancy, and the birth were obtained from the mothers of cases and controls, using a mailed self-administered questionnaire. A maternal age above 30 years was associated with odds ratios (OR) of 1.9 (95 percent confidence interval [CI]= 1.2-3.0) for cryptorchidism and 2.0 (CI = 1.2-3.6) for testicular seminoma; the latter effect was particularly high when the boy was the first child of the mother (OR = 4.1, CI = 1.1-14.6). Birth weights below 3,000 g or above4,000 g were associated with increased risks of testicular cancer, with OR sup to 2.6 (CI = 1.1-5.9) for birth weight below 2,500 g. For cryptorchidism, there was a monotonous trend in the OR from 0.4 in birth weights above 4,500 g to 2.3 in birth weights below 2,500 g. The association between cryptorchidism and testicular cancer was not attenuated by adjustment for maternal age and birthweight, indicating that all three variables are independent risk factors for testicular cancer. With the exception of high maternal age, which consistently is associated more strongly with seminoma than withnon-seminoma, it remains most likely that seminoma and non-seminoma have similar causes.