Fasting blood levels of three lipid classes (high-density lipoprotein-cholesterol [HDL-C], total cholesterol, and triglycerides) were measured in morbidly obese patients before, and six and 12 months after, Roux-en-Y gastric bypass. Weight above ideal in 31 women averaged 72 kg, and in 12 men, 78 kg. Excess weight lost after six months was 49.7%. Total cholesterol levels and abnormally high triglyceride levels (183 +/- 106 mg/dL [SD]) decreased and became normal after six months. Low-normal HDL-C levels in 31 women (45 +/- 13 mg/dL) improved by six months to 54 +/- 19 mg/dL; similarly, HDL-C levels in 12 men (38 +/- 9 mg/dL) increased to 57 +/- 17 mg/dL. One of eight diabetic patients remained hyperglycemic; ten of 15 patients who were hypertensive, requiring medication preoperatively, became normotensive by six months. Lipid levels measured again at one year showed little change. Risk factors of coronary heart disease were improved by gastric bypass in these morbidly obese patients, and the changes were clearly established by six months after operation.