We present our study of the genetics of undescended testes. The occurrence of undescended testes could be confirmed in 1.5 to 4.0 per cent of the fathers and in 6.2 per cent of the brothers of index patients with true undescended testes. The pattern of the familial cluster fits into the gaussian-additive-multifactorial-threshold program. The value of heritability for the first degree male relatives is 0.67 plus or minus 0.16. In siblings of index patients with undescended testes other congenital abnormalities do not occur more often. Bilateral (more severe) undescended testes are associated with a higher recurrence risk for siblings. The nosological identity of unilateral and bilateral undescended testes have shorter menses and a delay of menarche. Thus, the pituitary hypogonadism of mothers seems to be a predisposing factor for undescended testes in their sons.