Background Although assessment of progression of atherosclerosis by quantitative coronary angiography (QCA) is used as a surrogate for coronary events, no validation study has compared the several QCA measures used. Methods and Results The Cholesterol Lowering Atherosclerosis Study was a clinical trial testing the efficacy of colestipol-niacin on the progression of coronary atherosclerosis. Baseline/2-year coronary angiograms were obtained on 156 men with prior coronary artery bypass graft surgery. Changes in percent diameter stenosis and minimum lumen diameter (both measured in coronary lesions and segments) and coronary segment measures of average diameter, percent involvement, and vessel edge roughness were measured by QCA. Coronary events ascertained over 12 years of follow-up included myocardial infarction (MI), coronary death, and coronary artery revascularizations. Proportional hazards models evaluated the relation between QCA change measures and coronary events. Changes in percent diameter stenosis and minimum lumen diameter of coronary artery lesions were significantly related to the risk of MI/coronary death. All QCA measures were significantly related to the risk of any coronary event. Relative risks for each QCA measure were of similar magnitude when estimated separately within each treatment group. Change in minimum lumen diameter of lesions was the only measure independently associated with the risk of coronary events. Conclusions All QCA measures of progression of coronary artery disease were related to all coronary events (including revascularizations). Only QCA measures of lesion progression were related to MI/coronary death. QCA measures of lesion change may be better surrogate end points for “hard” coronary events than measures of change in coronary segments. (Am Heart J 2000;139:993-9.)