Summary— 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 < 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 anti-testis antibodies were absent. Testes prone to torsion already show impaired spermatogenesis.