BACKGROUND: Two potent weight loss therapies, a low-carbohydrate, ketogenic diet (LCKD) and orlistat therapy combined with a low-fat diet (O + LFD), are available to the public but, to our knowledge, have never been compared. METHODS: Overweight or obese outpatients (n = 146) from the Department of Veterans Affairs primary care clinics in Durham, North Carolina, were randomized to either LCKD instruction (initially, <20 g of carbohydrate daily) or orlistat therapy, 120 mg orally 3 times daily, plus low-fat diet instruction (<30% energy from fat, 500-1000 kcal/d deficit) delivered at group meetings over 48 weeks. Main outcome measures were body weight, blood pressure, fasting serum lipid, and glycemic parameters. RESULTS: The mean age was 52 years and mean body mass index was 39.3 (calculated as weight in kilograms divided by height in meters squared); 72% were men, 55% were black, and 32% had type 2 diabetes mellitus. Of the study participants, 57 of the LCKD group (79%) and 65 of the O + LFD group (88%) completed measurements at 48 weeks. Weight loss was similar for the LCKD (expected mean change, -9.5%) and the O + LFD (-8.5%) (P = .60 for comparison) groups. The LCKD had a more beneficial impact than O + LFD on systolic (-5.9 vs 1.5 mm Hg) and diastolic (-4.5 vs 0.4 mm Hg) blood pressures (P < .001 for both comparisons). High-density lipoprotein cholesterol and triglyceride levels improved similarly within both groups. Low-density lipoprotein cholesterol levels improved within the O + LFD group only, whereas glucose, insulin, and hemoglobin A(1c) levels improved within the LCKD group only; comparisons between groups, however, were not statistically significant. CONCLUSION: In a sample of medical outpatients, an LCKD led to similar improvements as O + LFD for weight, serum lipid, and glycemic parameters and was more effective for lowering blood pressure. TRIAL REGISTRATION: clinicaltrials.gov Identifier: NCT00108524.