The management of the undescended testis in the adult patient presents problems in terms of locating the impalpable gonad, the risk of malignancy and prognosis with regard to fertility. CT scanning has been used to locate four intra-abdominal testes. The risk of malignancy has been assessed by determining the incidence of carcinoma in situ after orchiectomy or testicular biopsy in 90 patients (112 testes). Carcinoma in situ was found in 4 of 16 abdominal, 3 of 44 inguinal and none of 52 scrotal testes following orchiopexy in childhood. Seventy per cent of undescended testes in adult patients had no evidence of spermatogenesis. This fact, together with the risk of malignancy in those testes in which germ cells are present, suggests that orchiectomy should be performed in unilateral cases. Testicular biopsy is essential if an undescended testis is to be preserved for androgenic function.