Testicular biopsies from 38 boys at the time of unilateral testicular torsion were studied retrospectively. Of the patients 30 were adolescent and 8 were prepubertal. In 18 boys biopsy of the torsed testis only was performed, while in another 18 bilateral testis biopsy was obtained. The remaining 2 patients had biopsy of the contralateral testis only because the torsed testis was infarcted completely. After allowances were made for the acute testicular changes associated with the torsion itself, significant pre-existing testicular abnormalities could be identified in 20 of the 38 patients (53 per cent), including the Sertoli-cell-only syndrome (7), partial Sertoli-cell-only syndrome (4), defective spermatogenesis (8) and mucous plugs caused by cystic fibrosis inducing tubular changes (1). In 70 per cent of the biopsies from prepubertal boys the number of spermatogonia per tubule was diminished. In adolescent boys the mean number of late spermatids also was diminished severely in the contralateral and torsed testes. Pronounced atrophy of Leydig cells was found in all but 1 testis examined. The presence or absence of a morphological blood-testis barrier did not appear to be related to pathological changes in the contralateral testis. These observations suggest that infertility in patients with unilateral testicular torsion may be a consequence of a pre-existing testicular pathological condition.