Testicular biopsies and hormone studies have been carried out on 229 children with unilateral (181) or bilateral (48) undescended ectopic obstructed testes not associated with other pathologic conditions. With regard to the histologic lesions, the obstructed testes may be classified into four types: (1) testes with minimal lesions (40.1%), showing slight reduction in both mean tubular diameter (MTD) and tubular fertility index (TFI); (2) testes with marked germinal hypoplasia (33.6%), showing slight or marked reduction in MTD and marked reduction in TFI; (3) testes with diffuse tubular hypoplasia (19.1%), showing severe reduction in MTD, marked or severe germinal hypoplasia, and normal or decreased Sertoli cell number per transverse tubular section (SCI); and (4) testes with Sertoli cell hyperplasia (7.2%), showing slightly decreased MTD, marked or severe germinal hypoplasia, and marked increased in SCI. These lesions are similar to those found in cryptorchid testes, although the proportion of testes with type III and IV lesions (the most severe) is lower than in cryptorchid testes. Hormone assays revealed normal basal gonadotropin and testosterone levels. The response of gonadotropins to gonadotropin-releasing hormone stimulation and the response of testosterone to human chorionic gonadotropin stimulation were normal or slightly reduced.