The effect of 1-wk administration of clofibrate on plasma glucose and insulin (IRI) before and during oral glucose tolerance tests (OGTT), as well as on serum lipids, uric acid, growth hormone (GH), and cortisol, were evaluated in 18 nondiabetic patients with hypertriglyceridemia and in 28 patients with chemical diabetes. Fasting plasma glucose, OGTT-glucose, and IRI areas were significantly decreased in both groups of patients, though the effects on glucose metabolism were much more marked in diabetics; 30-min IRI relative increase was unchanged; fasting plasma IRI was reduced in diabetics only. Glucose utilization during insulin tolerance tests carried out in 6 diabetics was significantly enhanced after treatment. Serum triglycerides (TG) and cholesterol (Chol) were significantly decreased in both groups of patients, as were serum free fatty acids and uric acid in diabetics; plasma GH and cortisol did not change. Significant correlations were found in diabetics between the postclofibrate decrease in OGTT-glucose area and the following: pretreatment values of serum Chol (r + 0.42, p less than 0.05) and of 30-min IRI absolute and relative increase (r + 0.44 and + 0.38, respectively, p less than 0.05); postclofibrate decreases in serum TG (r + 0.40, p less than 0.05), in fasting plasma glucose (r + 0.73, p less than 0.001), and in OGTT-IRI area (r + 0.57, p less than 0.01). These data suggest that the improvement in glucose metabolism observed during short-term clofibrate administration may be due to increased insulin sensitivity.