BACKGROUND: Obesity is a growing health concern in Canada and the United States, and pharmacologic therapies such as orlistat are being more commonly prescribed to assist with weight loss. OBJECTIVE: Our goal was to assess the efficacy and safety of orlistat compared with either placebo or an active control with regard to weight loss and serum lipid changes in overweight patients. DESIGN: We performed a systematic literature search of MEDLINE (1966 through December 2003) and the Cochrane Central Register of Controlled Trials. Relevant trials and reviews were searched by hand. Randomized trials comparing orlistat and a control and reporting changes in weight loss, serum lipids (total cholesterol, LDL cholesterol, HDL cholesterol, LDL:HDL, and triacylglycerols), or both in overweight and obese patients [body mass index (in kg/m2) > or =25] were included. RESULTS: Twenty-eight randomized trials met our inclusion criteria. Seventeen studies including 10,041 patients compared 3 x 120 mg orlistat/d with placebo or an inactive control along with a hypocaloric diet over a 1-y period. Relative risks (RRs) associated with clinically significant weight losses of 5% and 10% were 1.74 (95% CI: 1.57, 1.91) and 1.96 (1.74, 2.21), both favoring orlistat. Improvement in total cholesterol, LDL cholesterol, HDL cholesterol, and LDL:HDL were also greater with orlistat. Gastrointestinal events were more common with orlistat than with placebo [RR: 1.46 (1.37, 1.55)]. CONCLUSION: Our findings suggest that 3 x 120 mg orlistat/d is effective for improving both weight loss and serum lipid profiles in obese patients at low and high cardiovascular disease risk and in obese patients with type 2 diabetes.