Weight loss and consumption of viscous fibers both lower low-density lipoprotein (LDL) cholesterol levels. We evaluated whether or not a whole-grain, ready-to-eat (RTE) oat cereal containing viscous fiber, as part of a dietary program for weight loss, lowers LDL cholesterol levels and improves other cardiovascular disease risk markers more than a dietary program alone.Randomized, parallel-arm, controlled trial.Free-living, overweight and obese adults (N=204, body mass index 25 to 45) with baseline LDL cholesterol levels 130 to 200 mg/dL (3.4 to 5.2 mmol/L) were randomized; 144 were included in the main analysis of participants who completed the trial without significant protocol violations.Two portions per day of whole-grain RTE oat cereal (3 g/day oat b-glucan) or energy-matched low-fiber foods (control), as part of a reduced energy (∼500 kcal/day deficit) dietary program that encouraged limiting consumption of foods high in energy and fat, portion control, and regular physical activity.Fasting lipoprotein levels, waist circumference, triceps skinfold thickness, and body weight were measured at baseline and weeks 4, 8, 10, and 12.LDL cholesterol level was reduced significantly more with whole-grain RTE oat cereal vs control (−8.7±1.0 vs −4.3±1.1%, P=0.005). Total cholesterol (−5.4±0.8 vs −2.9±0.9%, P=0.038) and non–high-density lipoprotein-cholesterol (−6.3±1.0 vs −3.3±1.1%, P=0.046) were also lowered significantly more with whole-grain RTE oat cereal, whereas high-density lipoprotein and triglyceride responses did not differ between groups. Weight loss was not different between groups (−2.2±0.3 vs −1.7±0.3 kg, P=0.325), but waist circumference decreased more (−3.3±0.4 vs −1.9±0.4 cm, P=0.012) with whole-grain RTE oat cereal. Larger reductions in LDL, total, and non–high-density lipoprotein cholesterol levels and waist circumference were evident as early as week 4 in the whole-grain RTE oat cereal group.Consumption of a whole-grain RTE oat cereal as part of a dietary program for weight loss had favorable effects on fasting lipid levels and waist circumference.