Hypertrophic cardiomyopathy (HCM) is the most commonly diagnosed cardiomyopathy in cats. Cats with HCM typically have diffuse or focal thickening of the left ventricle of the heart. This may be associated with abnormal heart function that results in obstruction of blood ejection from the left ventricle, called hypertrophic obstructive cardiomyopathy (HOCM). HCM and HOCM may lead to congestive heart failure, arterial thromboembolism, fainting or sudden death. Numerous large clinical trials have demonstrated beneficial effects of beta-blocker agents (BBAs) in humans with congestive heart failure. There is no published evidence of clinical efficacy of BBAs in veterinary medicine. Carvedilol is a third generation BBA. Carvedilol has been demonstrated to improve left ventricular function, reduce infarct size and protect against lethal reperfusion injury in feline experimental models of regional myocardial ischemia. Two studies have demonstrated efficacy of BBAs in reducing left ventricular obstruction in cats with HOCM. However, no conclusive controlled clinical trials have been published that evaluate the efficacy and effect of BBAs on mortality in cats with naturally occurring cardiomyopathy and congestive heart failure. Therefore, the aims of this study are to evaluate the efficacy of carvedilol in cats with HCM, or HOCM, and congestive heart failure, and evaluate the effect of carvedilol on mortality in this population of cats.