Clinical and pathologic changes in 87 patients who could not be resuscitated from an episode of sudden cardiovascular collapse are described and compared with observations from patients in the same community who were successfully resuscitated from ventricular fibrillation. Findings in patients who died suddenly generally did not differ when the patients were groups by electrocardiographic rhythm on arrival of the mobile coronary aid unit. The pathologic changes of acute thrombosis and recent myocardial infarction did not occur with sufficient frequency in the entire group to be considered causally related to the sudden collapse, occurring in 10 and 5 percent of cases, respectively. Although most patients had evidence of obstructive coronary disease and old myocardial infarction, 8 percent had no significant vascular disease, acute thrombosis, myocarditis or valve disease that might be implicated as a factor in sudden death. There was no relation between age and severity of obstructive coronary disease or frequency of old myocardial infarction in patients who died suddenly. Complete atherosclerotic occlusion in one or more coronary vessels occurred in 51 of 87 (59 percent) and old myocardial infarction in 48 of 87 (55 percent). Although the mean age of this autopsy population was similar to that of all patients in the community who have had ventricular fibrillation on arrival of the aid unit, the nonsurvivors had a greater incidence of myocardial infarction and symptomatic heart disease (73 of 87) than did survivors. Comparison of this autopsy group with persons from the community who were resuscitated from ventricular fibrillation and subsequently had coronary angiograms indicates that the severity of coronary stenosis does not distinguish between survivors and nonsurvivors of an episode of ventricular fibrillation and suggests that other factors influence the outcome of an episode of ventricular fibrillation.