Seven hundred fifty-two cases of instantaneous sudden cardiac death were studied. Alcoholic cardiomyopathy was found in 127 cases (16.9%), predominantly in men under age 50 (73%). The heart was enlarged, with an average mass of 441 +/- 8 g. Light microscopy revealed uneven thinning and atrophy of the cardiomyocytes, with fatty infiltration and development of fatty tissue around the vessels of the left ventricle and interventricular septum, up to the subendocardial areas. In the majority of cardiomyocytes, electron microscopy showed atrophy of the myofibrils, pronounced dilation of the sarcoplasmic reticulum, lipids in the cytoplasm, mitochondriosis and an increased number of lysosomes and alterations of the mitochondria. Unchanged cardiomyocytes were characterized by hypertrophy and numerous ribosomes, and had developed a granular endoplasmic reticulum. Also observed were interstitial edema, an enlarged cardiomyocyte-capillary space, thinning of the capillary walls, flattening of the endothelium and a plethora and aggregation of erythrocytes. Decreases in mitochondrial enzyme activity were studied by enzyme histochemistry. Analogous pathologic changes may be individually present in other types of myocardial disease, but the complex of changes is specific for alcoholic cardiomyopathy. The frequency of alcoholic cardiomyopathy among victims of sudden death suggests that alcoholic cardiomyopathy is a risk factor for sudden cardiac death.