A placebo-controlled, double-blind study was performed to assess the effect of 12 weeks treatment with acipimox (250 mg three times per day) on lipoproteins and glycaemic control in patients with Type 2 diabetes. All patients studied had persistent hyperlipidaemia despite acceptable glycaemic control on treatment with diet alone or diet and oral hypoglycaemic agents, achieving glycosylated haemoglobin (HbA1) of less than 10.5% but with fasting total triglycerides greater than 2.5 mmol l-1 or total cholesterol greater than 6.5 mmol l-1. Forty-eight patients were randomized to treatment, 21 to acipimox and 27 to placebo; 43 completed the trial. All patients had been diabetic for at least 1 year. Total cholesterol fell by 6% and total triglycerides by 19% following 12 weeks of acipimox, compared to rises in the placebo group of 1% and 16%, respectively (p less than 0.05). There were no significant differences between acipimox and placebo in the change in low density lipoprotein (LDL) cholesterol, high density lipoprotein (HDL) cholesterol, apolipoproteins AI, AII, or B, or in glycaemic control during the treatment period. Acipimox is effective in reducing fasting total cholesterol and total triglycerides in patients with Type 2 diabetes with acceptable blood glucose control but persistent hyperlipidaemia. Acipimox does not adversely affect glucose tolerance.