We assessed the fertility of 40 patients who underwent orchiopexy between 1950 and 1960. Testicular biopsies also had been performed at operation, a practice exceptional for the era. Of 23 patients with unilateral undescended testes who attempted to have children 20 (87 per cent) were successful. In contrast, only 3 (33 per cent) of 9 patients with bilateral undescended testes fathered children. Over-all sperm counts in 16 patients were low but they were not predictive of paternity. Testicular biopsy specimens were reviewed and the fertility index was determined. In most cases a good correlation was found between histological status and paternity status.