A quantitative anaylsis of the degree and extent of coronary arterial narrowing by atherosclerotic plaques in the entire lengths of each of the 4 mjor epicardial coronary arteries in 22 patients with unstable angina pectoris is described at necropsy and the observations are compared to those in 20 control subjects. Of 1049 five-mm long segments of the left main, left anterior descending, left circumflex and right coronary arteries examined in the 22 patients (average 48 per patient), 497 (47%) were 76-100% narrowed in cross-sectional area by atherosclerotic plaques (controls equal 1%); 304 (29%) were 51-75% narrowed (controls equal 29%); 454 (48%) were 26-50% narrowed and only 119 (11%) segments were less than 26% narrowed (controls equal 22%).