OBJECTIVE In patients with azoospermia serum FSH helps to differentiate between obstruction or spermatogenetic dysfunction as the possible cause of this condition. The role of FSH in the diagnosis of infertile men with oligoasthenoteratozoospermia is less clearly defined. In order to evaluate the diagnostic significance of serum FSH in the management of male infertility, serum FSH levels were related to testicular morphology from bilateral biopsies of infertile men. METHODS Testicular biopsies were obtained from 213 infertile men and evaluated in semi-thin sections. Biopsies were performed either in order to distinguish between obstructive and non-obstructive azoospermia or because of subnormal semen variables when history, clinical investigation and hormone levels failed to explain infertility. Serum FSH was measured by fluoroimmunoassay. RESULTS Patients were divided into five groups on the basis of morphological criteria. The mean serum FSH value of patients with obstructive azoospermia and normal histology (group 1, n = 14) was normal (3.0 (2.2-4.1) IU/l) (mean (95% confidence limits)). Serum levels of FSH in non-obstructive oligo or azoospermia were as follows: group 2: mixed atrophy of tubular tissue without focal Sertoli cell only syndrome (SCO) (n = 104) (4.5 (4.0-5.1) IU/l), group 3: mixed atrophy with unilateral focal Sertoli cell only (n = 39) (7.4 (6.1-9.0) IU/l), group 4: mixed atrophy with bilateral focal SCO (n = 36) (10.7 (8.7-13.0) IU/l). Group 5: bilateral or unilateral total Sertoli cell only (n = 20) (16.0 (12.1-20.9) IU/l). Mean serum FSH levels were significantly different between all groups (P < 0.05). CONCLUSIONS Elevation of serum FSH correlates with the appearance of Sertoli cell only tubules. Elevated FSH serum levels make testicular biopsies superfluous for diagnostic purposes, but normal FSH does not exclude severe derangement of spermatogenesis in individual cases.