We studied four male patients with pituitary macroadenomas. Before treatment all had high serum FSH concentrations, but LH and testosterone were normal or subnormal; all patients were found to have large testes. All had had normal sexual function, and three patients had fathered children. After pituitary surgery there were decreases in serum gonadotrophins and testosterone, which were accompanied by decreases in testicular volumes. hCG stimulation tests in two patients showed normal responses of testosterone and oestradiol, confirming normal Leydig cell function. Inhibin levels were increased in two patients studied when FSH levels were high, suggesting a defect in gonadal-pituitary feedback control. Later, as FSH concentrations decreased to normal, so did inhibin levels. Histology showed that increased testicular size was due to increased lengths of seminiferous tubules. The association of pituitary macroadenomas, large testes and increased serum inhibin has not been reported previously. Assessment of testicular size in patients with raised serum FSH is important, since enlarged testes suggest the likely pathogenesis is that of a pituitary gonadotrophinoma, rather than primary gonadal failure. Increased inhibin levels may then confirm this, and be a biochemical marker for these tumours.