This study was undertaken to investigate the relationship of the serum sex hormone-binding globulin (SHBG) levels with the plasma insulin concentration and with the insulin resistance in male subjects with noninsulin-dependent diabetes mellitus (NIDDM). This investigation comprised 12 patients with NIDDM and 16 normal subjects matched for age, sex, and body mass index (BMI). There was a significant increase in insulin levels (P < 0.03) and a decrease in SHBG levels (P < 0.01) in the diabetic group as compared with those of the normal group. The sex hormone and plasma insulin levels were measured in NIDDM patients undergoing exercise and dietary therapy. Insulin sensitivity was evaluated by the hyperinsulinemic euglycemic clamp technique expressed as the glucose infusion rate (GIR) before and after the treatment. The SHBG levels correlated significantly with the insulin concentrations (r = -0.643, P < 0.05) and with the GIR (r = 0.615, P < 0.05) before the treatment. The SHBG levels (P < 0.02) and GIR (P < 0.01) increased, and the insulin concentrations (P < 0.01) decreased significantly during the treatment. The SHBG levels showed a negative and significant correlation with the plasma insulin concentrations at the end of the clamp study before (r = -0.615, P < 0.05) and after (r = -0.626, P < 0.05) the treatment. These findings suggest that, in the hyperinsulinemic state, plasma insulin has a direct effect on the SHBG levels. SHBG levels decreased significantly during the clamp study before (P < 0.02) and after (P < 0.01) the treatment. This may represent the acute effect of insulin on the SHBG levels. Briefly, these results suggest that insulin may directly affect the SHBG levels and that SHBG may constitute an index of the insulin resistance only in the hyperinsulinemic state.