Cryptorchidism is a feature of abnormalities in the hypothalamo-pituitary-testicular axis, and almost all disorders of sexual differentiation in which a testis is present. We found cryptorchidism to be associated with malformations and dysplasias of the kidneys, the ureters and the spine from T10 to S5. The description of this association is new. The association was seen in 18% of cryptorchid boys younger than 3 years of age in a department of paediatric surgery, in 34% of cryptorchid foetuses who died in the third trimester, in 65% of cryptorchid patients with imperforate anus, and in all individuals with tritonmelia, the male variant of sirenomelia. Sirenomelia/tritonmelia is an extreme degree of abnormal differentiation of the caudal developmental field, also called caudal dysplasia, the caudal regression syndrome and the caudal regression malformation sequence. Caudal developmental field defects were also the predominant abnormalities in the other groups of patients. Thus, cryptorchidism may be a feature of abnormal differentiation of the caudal developmental field. Position and histology of the undescended testes of the patients included in the association were similar to in cryptorchidism in general. In the literature the association was reported in 5-10% of boys considered to suffer from cryptorchidism only. Furthermore, our observations are concordant with recent theories about cryptorchidism. Consequently, we propose that cryptorchidism in general may be a caudal developmental field defect. Study of cryptorchid patients exhibiting malformations or dysplasias of the kidneys, the ureters or the spine from T10 to S5 is essential in order to isolate new genetic disorders and to spot environmental factors causing cryptorchidism.