In an attempt to explain the oligozoospermia commonly observed after unilateral testicular torsion, 56 patients with acute torsion were investigated prospectively. Blood was taken pre-operatively for antibody studies and a contralateral testicular biopsy was performed at the time of orchiopexy. At review 3 to 6 months post-operatively, late testicular atrophy was assessed and repeat antibody studies were performed together with a hormone profile and seminal analysis. Although the duration of torsion showed a close correlation with the degree of testicular atrophy (P less than 0.001), no such association could be demonstrated between duration of torsion and subsequent sperm concentration. Twenty of 35 patients had histological evidence of a pre-existing partial maturation arrest in spermatogenesis. Fifteen of 19 with the abnormality were oligozoospermic, while all of those with normal testicular histology had a sperm concentration within the normal range. Anti-sperm antibody formation following torsion was minimal and antitestis antibodies were absent. Testes prone to torsion already show impaired spermatogenesis.