The effects of colestipol (30 grams/day), niacin (7.3 grams/day), and diet on blood lipids and apolipoproteins after one year of therapy are reported. Men selected on the basis of previous coronary artery bypass surgery were randomly assigned to drug or control treatments in an angiographic study of atherosclerosis progression and regression. In 14 men, drugs and diet produced the following changes: Baseline total cholesterol 245 mg/dl, triglyceride 189 mg/dl, and LDL cholesterol 164 mg/dl were decreased by 73 mg/dl (29%), 83 mg/dl (41%) and 69 mg/dl (40%) respectively. Baseline HDL cholesterol, 44 mg/dl was increased 13 mg/dl (33%). Baseline apolipoprotein B, 124 mg/dl and apolipoprotein C-III (heparin precipitate) 5.6 mg/dl were decreased 40 mg/dl (31%) and 2.4 mg/dl (41%) respectively. All these changes are significant, p less than 0.01. Apolipoprotein A-I and apolipoprotein C-III (heparin supernate) were not significantly changed. In the controls, placebo and diet produced no significant decrease in blood lipid or lipoproteins, with the exception that baseline apolipoprotein B, 111 mg/dl increased 18 mg/dl (12%), p less than 0.05.