BACKGROUND Spontaneously occurring small animal models of myocardial disease, closely resembling the human condition, have been reported for hypertrophic cardiomyopathy (in cats) and arrhythmogenic right ventricular cardiomyopathy (in cats and boxer dogs). Nonhypertrophied restrictive cardiomyopathy (RCM) is a well-recognized but relatively uncommon primary heart muscle disease causing substantial morbidity in humans. We describe RCM occurring in felines here as a potential model of human disease. METHODS We used two-dimensional and Doppler echocardiography to define morphologic and functional features of RCM in 35 domestic cats (25 male; 10±4 years old) presenting to a subspecialty veterinary clinic. Ten underwent complete necropsy examination. Echocardiographic parameters of diastolic filling were compared to those in 41 normal controls. RESULTS The 35 cats presented with congestive heart failure (n=32), lethargy (n=2), or syncope (n=1), associated with thromboembolism in 5 and supraventricular tachyarrhythmias in 8. During an average 4.4-year follow-up period, 18 died or were euthanized due to profound heart failure, and 3 died suddenly; survival from clinical presentation to death was 0.1 to 52 months. Echocardiographic and necropsy examination showed biatrial enlargement, nondilated ventricular chambers, and normal wall thicknesses and atrioventricular valves. Histopathology demonstrated disorganized myocyte architecture and patchy replacement myocardial fibrosis. Pulsed Doppler demonstrated restrictive physiology with increased early (E) mitral filling velocity (1.1±0.3 m/s) and peak E to peak late (A) flow ratios (4.3±1.2), reduced A filling velocity (0.3±0.1 m/s), and shortened mitral deceleration time (40.7±9.3 ms; all P<.001 vs. controls), with preserved left ventricular systolic function. CONCLUSIONS A primary myocardial disease occurring spontaneously in domestic cats is remarkably similar to restrictive nondilated and nonhypertrophied cardiomyopathy in man and represents another potential animal model for human disease.