Cryptorchidism is the absence of at least one testis in the scrotum. The incidence of cryptorchidism is high and almost 3% of boys in the western countries are operated on for this condition. Abnormalities in the hypothalamo-pituitary-testicular axis may result in cryptorchidism, and cryptorchidism is nearly almost present in individuals with a testis and abnormal sexual differentiation. We characterized an association of cryptorchidism and T10-S5 vertebral, renal and ureteral malformations and dysplasias. Cryptorchidism was ipsilateral to renal and ureteral malformations and dysplasias in more than 90% of cases. The association was proved with increasing frequency in: 4-10% of cryptorchid boys who had undergone laparoscopy for impalpable testes or were expected to be without additional abnormalities; 18% of cryptorchid boys in a material from a department of pediatric surgery; 34% of cryptorchid individuals who had died between week 28 and 40 of gestation; 65% of cryptorchid boys operated on for imperforate anus; all individuals with tritonmelia who are always cryptorchid. As the association was demonstrated with increasing frequency the higher the degree of abnormal differentiation of the caudal developmental field, cryptorchidism may be interpreted as a feature of such an abnormal differentiation. The general significance of our hypothesis is supported by the fact that both position and histology of the undescended testes were independent of a possible presence of the malformations and dysplasias encompassed by the association. Intra-abdominal testes were associated with persisting diaphragmatic ligaments which again were associated with an abnormal differentiation of the midline. Consequently we consider cryptorchidism a feature of abnormalities in the hypothalamo-pituitary-testicular axis and of an abnormal differentiation of the male sexual organs, the midline, and/or the caudal developmental field. The recommended age at surgery for cryptorchidism has gradually been lowered. The recommendations are mainly based on analyses of number of spermatogonia per tubular transverse section in normal and undescended testes. In 1984 when we started our investigations, the biological value of this variable was unclarified.