A 9-week-old Weimaraner was tentatively diagnosed clinically with congenital left-sided heart failure. On necropsy, diffuse fibroplasia over the left ventricular endocardium; small, deformed papillary muscles; and pulmonary congestion were evident. Microscopically, the fibroplasia extended into the myocardium and cardiomyocytes, and Purkinje fibers were degenerated, consistent with congenital endocardial fibroelastosis.