In 62 patients with coronary disease who had serial arteriograms without intervening coronary artery bypass graft (CABG) or percutaneous transluminal coronary arteriography (PTCA), progression was seen in 48 (77 percent). Progression from a normal or minimally narrowed lumen diameter to narrowing greater than or equal to 75 percent (to greater than or equal to 90 percent) in 21 patients) occurred in at least one vessel in 33 patients (69 percent) (group A, type I progression). Less striking progression and progression of initially more severe lesions was seen in 15 of 29 patients without type 1 progression (Group B) and in other vessels in 12 group A patients. Improvement in at least one vessel was seen in eight patients. There was no difference between groups A and B in the incidence of risk factors, intervening myocardial infarction, or recent unstable angina. It is concluded that progression of occlusive coronary disease occurs as commonly in areas of the coronary tree that are minimally diseased as in segments that are initially severely narrowed. Methods to stabilize the endothelium may prevent progression of coronary artery disease.