Five hundred consecutive coronary artery segments histologically narrowed 76 to 95% in cross-sectional area by atherosclerotic plaque were classified into concentric and eccentric luminal types. Of the 500 segments, 365 (73%) were eccentric and 185 (27%) were concentric. Of the eccentric lesions, an arc of disease-free wall accounted for 2.3 to 32% (mean 16.6%) of the circumference of the internal elastic membrane. The average thickness of the coronary artery media was thinner in diseased segments (mean 88.4 micron) compared to disease-free wall segments in the same vessel (mean 202.9 micron). The presence of disease-free wall segments has clinical importance in at least three areas: (1) the accuracy with which angiography reflects the morphologic features of the diseased vessel, (2) coronary artery spasm, and (3) the mechanism of coronary balloon angioplasty and subsequent "restenosis" of previously dilated vessels.