In vitro studies have shown that the Sertoli cell is the primary source of inhibin in the male. To assess the regulation of inhibin production we measured serum inhibin, FSH, LH and testosterone in 21 normal men and 104 men with various testicular disorders resulting in infertility. The infertile men were subdivided into groups on the basis of their mean sperm count, FSH and LH levels. The mean serum concentrations of inhibin in the normal men were 602 +/- 29 U/L and were significantly decreased in those groups with severe oligozoospermia (p < 0.01) or azoospermia (p < 0.01; p < 0.001). The FSH concentrations correlated inversely with serum inhibin concentrations (p < 0.001) in azoospermic men. Azoospermic men with high FSH had significantly lower inhibin and testosterone levels when compared with normal men (p < 0.01; p < 0.001). Serum FSH concentrations were significantly increased in azoospermic men (p < 0.001). Our present results of serum concentrations of inhibin correlating inversely with those of FSH levels suggest that measurement of inhibin may be a useful circulating marker of Sertoli cell function.