Sudden death with severe dyspnea in new-born or infant calves confined to a family of Japanese black cattle was studied. Neither environmental nor bacteriological factors seemed to be involved in the etiology of the disease. Histopathological examination revealed focal degeneration and necrosis of the muscle fibers in the myocardium, to which a diagnosis of idiopathic cardiomyopathy was proposed. The pedigree and mating type analyses suggested that a new autosomal recessive lethal gene was responsible for the syndrome. Rates of incidence of the affected calves in five sire families were tested for an expectation of 12.5 percent incidence by the chi-square method.