Testicular biopsy specimens from 4 (8%) of 50 men previously treated for maldescended testes had a carcinoma-in-situ pattern. 2 of these men had adjacent invasive carcinoma (seminoma and embryonal carcinoma). The patient with embryonal carcinoma had an enlarged testis but the 3 other patients with neoplasia had no clinical signs or symptoms of malignancy. Routine follow-up including testicular biopsy in young men with maldescended testes is essential because of the increased risk of malignancy and may detect testicular neoplasia at a stage when orchidectomy alone is curative.