Familial cryptorchidism is a poorly understood and infrequently reported condition. We have had the opportunity of studying three different families. In the first family, the father, his twin 3-year-old sons, and their 6-year-old sibling had undescended left testes. In the second family, left-sided cryptorchidism occurred in the father, his brother, and his 7-year-old son. In the third family, twin brothers, 2 years of age, had cryptorchidism on the left side. There was no family history of cryptorchidism. In all cases the testes were located in the inguinal canal.