Hyperlipidaemia, in particular raised concentrations of serum triglycerides, together with raised plasma non-esterified fatty acid concentrations, is common in patients with Type 2 (non-insulin-dependent) diabetes mellitus and may be associated with insulin insensitivity. Thirty non-obese Type 2 diabetic patients (15 controlled with diet alone and 15 with diet plus oral sulphonylurea therapy) were therefore recruited to take part in a double-blind, randomized, crossover comparison of acipimox (250 mg three times daily for 3 months) and placebo. Serum lipids, blood glucose control, insulin sensitivity, and glucose tolerance were measured before and after each treatment period. There was a significant decrease in serum triglycerides (2.05 +/- 1.08 vs 2.91 +/- 1.75: p < 0.005), cholesterol (5.66 +/- 1.02 vs 6.26 +/- 1.17: p = 0.0005), and apoprotein B (1.32 +/- 0.23 vs 1.44 +/- 0.25: p < 0.05) while HDL cholesterol and apoprotein A-1 concentrations were unchanged. There was no change in blood glucose control measured by fasting glucose, insulin, and HBA, concentrations, but there was a significant improvement in insulin action assessed by glucose-insulin infusion. Although plasma non-esterified fatty acid concentrations were lower during the oral glucose tolerance test after acipimox, there was no difference in either the peak or 2-h plasma glucose concentrations and the total area under the glucose curve did not change. Acipimox was well tolerated and no patients withdrew from the study for drug-related symptoms. Thus, acipimox effectively lowers serum cholesterol and triglycerides in patients with Type 2 diabetes without adversely altering blood glucose control, and appears to improve insulin sensitivity.