The morphologic effects of percutaneous transluminal coronary artery balloon angioplasty (PTCA) on atherosclerotic vessels is described in six patients who died at varying intervals after the procedure (four early and two late). In the early group (less than one week post PTCA) one patient died because of electromechanical dissociation during emergency PTCA for evolving infarct; in the three other patients PTCA was performed for left main occlusion and cardiogenic shock with deaths 3, 24 and 25 h after PTCA. The two late deaths were patients who died one and nine months after PTCA from unrelated causes. There were 12 sites of balloon inflation in the six patients, all in left main, isolated marginal or left anterior descending arteries. Post mortem examinations, with in toto serial sectioning of the ballooned coronary arteries, revealed a number of local morphologic changes. Plaque fractures and disruptions of the arterial wall to variable depths were observed. At four sites these fractures were through media, and at one site was associated with a large dissection. These cases had only small epicardial hemorrhages or reactive adventitial changes associated with these deep fractures. Four of the six patients had intramural arterial emboli (athero/thrombo/calcium/foreign body). These findings confirm that a large part of the effect of PTCA is due to physical disruption of plaque and underlying native vessel.