Abnormalities in lipoprotein metabolism are among the risk factors for atherosclerotic cardiovascular disease frequently present in patients with morbid obesity. We have examined the effects of dietary restriction induced by gastric bypass surgery on plasma lipoprotein levels in 22 morbidity obese patients. Operation induced weight loss in all patients. Postoperatively (12 +/- 7 mo), triglycerides decreased from 146.4 +/- 67 (mean +/- S.D. before operation) to 104 +/- 44 mg/dl (p less than 0.01), total cholesterol decreased from 187 +/- 52 to 166 +/- 39 mg/dl (p less than 0.025), LDL cholesterol levels decreased from 119 +/- 42 to 104 +/- 36 mg/dl (p less than 0.038), and HDL cholesterol levels increased from 40.1 +/- 10.1 to 45.2 +/- 9.5 mg/dl (p less than 0.012). HDL cholesterol to LDL cholesterol ratio increased from 0.37 +/- 0.13 to 0.48 +/- 0.21 (p less than 0.01). The results of apo A-1, A-2 and B determinations (n = 12) paralleled the changes in lipoprotein levels. Apo A1/Apo A2 ratio increased suggesting an increase in the concentration of HDL2. We conclude that weight loss following gastric bypass surgery is associated with favorable changes in lipoprotein metabolism which may help prevent or delay the development of atherosclerotic cardiovascular disease. The mechanism responsible for the lipoprotein changes remains to be determined.