Postmortem coronary angiographic morphology was correlated with histologic sections of 73 localized subtotal coronary artery stenoses (50-99% reduction of luminal diameter) to determine whether complicated or uncomplicated atherosclerotic lesions could be detected angiographically. Lesions were divided into two types, according to angiographic morphology: Type I stenoses had smooth borders, an hourglass configurations, and no intraluminal lucencies; type II stenoses had irregular borders or intraluminal lucencies. Histologic sections were also divided into two types: "uncomplicated" stenoses had fatty or fibrous plaques with intact intimal surfaces and no superimposed thrombus; "complicated" stenoses manifested plaque rupture, plaque hemorrhage, superimposed partially occluding thrombus, or recanalized thrombus. Among 35 lesions with type I angiographic morphology, four (11.4%) were complicated lesions histologically. Among the 38 stenoses showing type II angiographic morphology, 30 (78.9%) were complicated lesions. Postmortem angiography thus had a sensitivity of 88% and specificity of 79% for detecting complicated stenoses on the basis of irregular borders or intraluminal lucencies. Pathologic studies have shown that acute occlusive thrombosis of a coronary artery is usually associated with complicated atherosclerotic stenoses. Thus, complicated lesions represent a greater risk factor for acute myocardial infarction or sudden death than do uncomplicated lesions. This study suggests that coronary stenoses characterized angiographically by irregular borders or intraluminal lucencies are probably the clinically more dangerous "complicated" type.