Cryptorchism was diagnosed in all 3 male members of 2 pairs of dizygotic twins in a sibship. Two of these brothers developed seminomas at age 31 and 33 years. Studies revealed a nephew with an atrophic testicle, but no additional instances of twinning, cryptorchism or testis cancer. Cryptorchism has been reported previously in 3 of 8 sets of twins with testis cancer, but rarely in familial testis cancer affecting other relatives. Hormonal factors may be involved in the association of twinning, cryptorchism and testis cancer in this family.