Time, the severity of preexisting arterial stenosis, and coronary-artery surgery affect the progression of coronary atherosclerosis. As plaque formation worsens, the probability of acute obstruction increases significantly in medical and surgical patients.1 No clinical or laboratory risk factors consistently predict coronary arterial progression. The course of disease in arteries narrowed less than 50 per cent is less predictable but accounts for a substantial amount of progression documented by sequential coronary arteriography.2 Progression develops at nearly identical rates in unbypassed arteries in surgical patients and in medical patients with comparable lesions.3 The lowest rate of progression occurs in undiseased segments. The . . .