A 2-year-old Old English Sheepdog had right-side congestive heart failure characterized by pericardial effusion, pleural effusion, ascites, and increased pulmonary wedge pressure. A diagnosis of atrial septal defect was made by means of cardiac catheterization and angiography. Surgical correction initially was deferred because of pulmonary hypertension. However, when congestive heart failure could not be managed adequately, surgical correction was attempted. Postsurgically, thrombocytopenia developed and that led to bleeding, oliguira, and pulmonary edema. The bleeding was controlled by whole blood transfusion, but the oliguria and pulmonary edema were not resolved, even with intensive diuretic therapy. The dog died 24 hours after surgery.