The effect of orlistat, a nonabsorbed inhibitor of gastric and pancreatic lipases, was examined in patients with primary hyperlipidaemia (serum cholesterol > or = 6.2 mmol.l-1 and triglycerides < or = 5.0 mmol.l-1) not responsive to dietary change alone. In a multicentre, randomised, double-blind study, 103 men and 70 women received 30, 90, 180, or 360 mg or orlistat or placebo for 8 weeks. Total and low-density lipoprotein cholesterol levels were reduced by 4% and 5% with 30 mg orlistat, by 7% and 8% with 90 mg orlistat, by 7% and 7% with 180 mg orlistat and by 11% and 10% with 360 mg orlistat compared to placebo. High density lipoprotein cholesterol levels significantly decreased in the 360 mg orlistat group. Triglyceride levels significantly increased in the placebo group but not in the drug groups. Body weight decreased by 1.2 kg with 360 mg orlistat, despite a weight maintenance diet. Decreases in vitamin E and D levels occurred, although both vitamins remained within the normal range. Adverse effects from the gastrointestinal tract were frequent, but led to discontinuation of therapy in only seven patients. Orlistat is a new therapeutic drug for the treatment of hyperlipidaemia that may be particularly useful among overweight patients. Its potential place in therapy will await long-term studies. Vitamin supplementation should be considered during treatment.