In this case--control study of 108 cases of testicular cancer in men under 30 years of age, cryptorchidism was a major risk factor [relative risk (RR) = 9.0]. Low birth weight was also associated with increased risk (RR = 3.2). Having severe acne at puberty was protective (RR = 0.37). Interviews with mothers of cases revealed that exposure of the mother to exogenous estrogen during pregnancy created a significant risk in the son (RR = 8.0). In first pregnancies, excessive nausea indicated an increased risk of testicular cancer (RR = 4.2). Increased body weight in the mother also increased the risk. The relation between these factors and testicular hypoplasia is discussed. Severe perimenopausal menorrhagia was a factor in the mother associated with reduced risk of testicular cancer in the son (RR = 0.10). A modified hormonal milieu in the mother appears to be important in the later development of testicular cancer in her sons.