OBJECTIVE: The purpose of this study was to assess the effect of orlistat, a gastrointestinal lipase inhibitor, on body weight, glycemic control, and cardiovascular risk factors in metformin-treated type 2 diabetic patients. RESEARCH DESIGN AND METHODS: A 1-year multicenter, randomized, double-blind, placebo-controlled trial of 120 mg orlistat t.i.d. (n = 249) or placebo (n = 254) combined with a reduced-calorie diet was conducted in overweight and obese patients with suboptimal control of type 2 diabetes. RESULTS: After 1 year of treatment, mean (+/-SE) weight loss was greater in the orlistat than in the placebo group (-4.6 +/- 0.3% vs. -1.7 +/- 0.3% of baseline wt, P < 0.001). Orlistat treatment caused a greater improvement in glycemic control than placebo, as evidenced by a greater reduction in serum HbA(1c), adjusted for changes in metformin and sulfonylurea therapy (-0.90 +/- 0.08 vs. -0.61 +/- 0.08, P = 0.014); a greater proportion of patients achieving decreases in HbA(1c) of > or = 0.5 and > or = 1.0% (both P < 0.01); and a greater reduction in fasting serum glucose (-2.0 +/- 0.2 vs. -0.7 +/- 0.2 mmol/l, P = 0.001). Compared with the placebo group, patients treated with orlistat also had greater decreases in total cholesterol, LDL cholesterol, and systolic blood pressure (all P < 0.05). Although more subjects treated with orlistat experienced gastrointestinal side effects than placebo (83 vs. 62%, P < 0.05), more subjects in the placebo group withdrew prematurely from the study than in the orlistat group (44 vs. 35%, P < 0.05). CONCLUSIONS: Orlistat is a useful adjunctive treatment for producing weight loss and improving glycemic control, serum lipid levels, and blood pressure in obese patients with type 2 diabetes who are being treated with metformin.