A case of idiopathic dilated cardiomyopathy was studied histologically and by quantitative determination of tissue lipid concentration in order to document increased lipid accumulation. Four chambers of the heart were dilated with left ventricular dominance. Microscopically, there was a mixture of primarily fragmented myocytes with sarcoplasmic degeneration and minorly hypertrophic myocytes. There were numerous areas of moth-eaten appearance of myocardium in all of the sections, which corresponded to fatty degeneration, composed of numerous small lipid droplets between decreasing contractile elements. Ultrastructurally, there were widened Z-bands in degenerated myocardium, corresponding to light microscopically enlarged contraction bands. The extent of moth-eaten appearance was more extensive in the left than in the right ventricle, which was also verified by a three-fold increase of triglyceride concentration in the left ventricle compared with age-matched control hearts. The moth-eaten appearance was also observed in the auricles and atria with less immunohistochemical staining intensity for atrial natriuretic polypeptide, demonstrating that this fatty change involves all parts of the myocardium.