To explore risk factors for testicular cancer and cryptorchidism, two 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 two hospital series of men treated for cryptorchidism and a control group sampled among residents in the Copenhagen area. The 2,037 men were interviewed by telephone. The relative risk (RR) of testicular cancer in men with treated or persisting cryptorchidism was 3.6 (95 percent confidence interval = 1.8-6.9), but no increase in risk was seen in the six to seven percent of the men who reported a history of undescended testes that descended spontaneously. The RR in men who were treated for cryptorchidism increased with age at treatment. This effect may be due wholly or in part to increased treatment of boys with testes that would have descended spontaneously if they had not been treated. Cryptorchidism and inguinal hernia may be confused and reported interchangeably. In the absence of cryptorchidism or testicular atrophy, clinical inguinal hernia was not associated with testicular cancer. Testicular atrophy was associated with both testicular cancer and cryptorchidism. Associations with other congenital malformations were few and based on small numbers.