To explore risk factors for testicular cancer and cryptorchidism, 2 parallel case-control studies were conducted in Denmark. The testicular-cancer study was population-based and included 514 cases and 720 controls. The cryptorchidism study included 387 cases and 416 controls and was based on 2 hospital series of men treated for cryptorchidism and a control group sampled among residents in the Copenhagen area. The 2037 men were interviewed by telephone, and self-administered questionnaires were sent to their mothers. A strong association was seen between low social class and cryptorchidism, with sons of unskilled workers having a 3-fold higher risk of cryptorchidism than sons of self-employed men. Testicular cancer was only moderately associated with high-social-class indicators, and only with such indicators pertaining to the mother. Both testicular cancer and cryptorchidism tended to occur more frequently in first-born men and in sons of older women but these associations were not statistically significant. Late puberty was associated with reduced risk of testicular cancer. The effect of age at puberty may be due both to advanced age at diagnosis and to the existence of common determinants of age at puberty and testicular cancer. Men who had been treated for cryptorchidism entered puberty later than other men, possibly because of impaired hormonal function of the testes. There was no indication of increased risk of testicular cancer or cryptorchidism in sons of mothers who smoked around the time of conception or during the pregnancy.