The effects of clofibrate treatment have been monitored in a double-blind cross-over study conducted in 16 male patients with coronary heart disease. Most had latent diabetes mellitus with elevated and delayed insulin release after i.v. glucose administration. Blood glucose and insulin levels were measured during repeated i.v. glucose tolerance tests in each patient and serum triglyceride and plasma fibrinogen were estimated at intervals. Clofibrate treatment significantly lowered fasting blood glucose levels (p less than 0.01) and improved the glucose tolerance (p less than 0.01). Fasting plasma insulin levels and those at 30 min after glucose loading were reduced (p less than 0.05). Serum triglycerides (p less than 0.01) and plasma fibrinogen levels (p less than 0.05) were lowered during the treatment period. The change in k-value (glucose utilization) did not correlate to changes in triglyceride or fibrinogen. This study confirms the beneficial effect of clofibrate therapy on abnormal glucose tolerance observed by other workers. It is suggested that clofibrate acts by reducing peripheral insulin resistance.