Excessive numbers of moderator bands bridging the left ventricular septum and free wall and entangling papillary muscles were associated with heart failure and death in 21 cats. Clinical findings included dyspnea, anorexia, hypothermia, cardiomegaly, pleural effusion, plumonary edema, heart murmurs, gallop rhythm, electrocardiographic abnormalities (especially conduction disturbances), increased left ventricular end-diastolic pressure, angiocardiographic evidence of left ventricular restriction, and aortic thromboembolism. Pathologic changes included a morphologically distinct network of abnormal numbers of moderator bands in the left ventricle, left ventricular hypertrophy (younger cats--mean age, 4 years) or dilatation (older cats--mean age, 8.7 years), left atrial enlargement and hypertrophy, and pulmonary edema with heart failure cells in the alveoli. Heart weights of affected cats were significantly less than those of cats with congestive, hypertrophic, and restrictive cardiomyopathy (endocardial fibrosis), but were not significantly less than heart weights of clinically normal cats. Pathologic changes were characteristic of the syndrome grossly and histologically, but clinical findings were not clearly definable.