Bilateral testicular biopsies and clinical histories from 34 adult men with maturational arrest of spermatogonia were examined. According to the morphology of Sertoli cell nuclei, five testicular types of spermatogonial maturational arrest were established. In type I lesion, Sertoli cells resembled the immature Sertoli cells of infant testes. These cells had a round, regularly outlined, dark nucleus with a small nucleolus. The seminiferous tubules showed no apparent lumen and a poorly developed lamina propria lacking in elastic fibres. This lesion was found in patients exhibiting a eunuchoid phenotype, with small tests and low serum levels of gonadotrophins and testosterone (hypogonadotrophic hypogonadism). Type II lesion showed morphologically normal, mature, adult Sertoli cells which had a pale, irregularly outlined nucleus, many often triangle-shaped, with a large, centrally located nucleolus. The seminiferous tubules were reduced in diameter and showed a few spermatocytes and spermatids. This lesion was found in patients with varicocoele, epididymitis, testicular trauma or idiopathic infertility. Serum FSH levels were normal or increased while LH and testosterone levels were normal. In type III lesion, Sertoli cells resembled the involuting Sertoli cells found in the testes of aging men, and displayed very infolded nuclei, with abundant dense chromatin patches and a large nucleolus. The seminiferous tubules showed a slightly dilated lumen and a normal tubular wall. The most relevant clinical findings in patients with this lesion were alcoholism, varicocoele, falciform cell anaemia, epididymitis and germ cell tumour. Serum follicle stimulating hormone (FSH) levels were normal or increased while luteinizing hormone (LH) and testosterone levels were normal. Type IV lesion Sertoli cells presented with a de-differentiated appearance. These cells had a small, round euchromatic nucleus with a small nucleolus and vacuolated cytoplasm. The seminiferous tubules were devoid of lumen or ectatic, and the tubular wall was thick and contained abundant elastic fibres. This lesion was characteristic of patients who underwent hormonal treatment because of prostatic carcinoma or sex change. Type V lesion showed abnormally differentiated, probably dysgenetic, Sertoli cells which had a round to ovoid regularly outlined nucleus, with small heterochromatin granules, and the number of these cells was increased. The seminiferous tubules had a central lumen, or were ectatic with vacuolated Sertoli cells, and the amount of elastic fibres was decreased. The most relevant clinical finding in patients with this lesion was orchidopexy. Serum FSH and LH levels were normal or slightly increased. These findings indicate that spermatogonial maturational arrest is associated with a characteristic Sertoli cell morphology that can be easily identified. This morphology may shed light on the aetiology of the disorder, and be useful for establishing the prognosis and bases for treatment in subfertile patients.