Beneficial effects of colestipol-niacin on coronary atherosclerosis. A 4-year follow-up. The Cholesterol Lowering Atherosclerosis Study (CLAS) was a randomized, placebo-controlled, angiographic trial testing combined colestipol-niacin therapy in 162 subjects. Two-year results (CLAS-I) showed decreased atherosclerosis progression and increased regression. We now describe a subgroup of 103 subjects treated for 4 years (CLAS-II). Changes in blood lipid, lipoprotein-cholesterol, and apolipoprotein levels were maintained, and at 4 years significantly more drug-treated subjects demonstrated nonprogression (52% drug- vs 15% placebo-treated) and regression (18% drug- vs 6% placebo-treated) in native coronary artery lesions. Significantly fewer drug-treated subjects developed new lesions in native coronary arteries (14% drug- vs 40% placebo-treated) and bypass grafts (16% drug- vs 38% placebo-treated). These results confirm CLAS-I findings and indicate that regression can continue for 4 years. They reaffirm the need for early initiation of vigorous long-term lipid lowering therapy in coronary bypass subjects. 