Serum concentrations of inhibin, FSH and LH were measured in 39 normal men and 127 men with testicular disorders resulting in infertility. The infertile men were divided into groups on the basis of their mean sperm count, FSH levels and karyotype. The mean (±s.d) serum concentrations of inhibin in the normal men was 554 ± 156 U/l and did not differ significantly from those groups with oligospermia, azoospermia or Klinefelter's syndrome. Combined analyses of all groups did not reveal any significant correlation between serum concentrations of inhibin and FSH or with any other parameter measured. Serum concentrations of FSH and LH were positively correlated, and Leydig cell dysfunction, as evidenced by increased serum LH levels, low testosterone levels or a declining testosterone/LH ratio were found with severe spermatogenic damage. The failure of serum concentrations of inhibin to correlate with those of FSH levels or the degree of testicular damage raise questions as to the clinical value of this parameter alone. Journal of Endocrinology (1989) 120, 517–523