The endoventricular biopsies of nine patients with alcoholic congestive cardiomyopathy were studied under the light microscope and under the electron microscope. These were compared with the results of nine cases of primary congestive cardiomyopathy, at a similar clinical stage. In the absence of specific signs, the overall appearance on both light and electron microscopy is sufficiently suggestive to allow differentiation of the two forms of cardiomyopathy. Repeated biopsies revealed disappearance of the nuclear abnormalities after withdrawal of alcohol, which introduces a prognostic element.