The caval syndrome is a serious complication of chronic heartworm (Dirofilaria immitis) disease in dogs and cats. The syndrome is characterized by acute anorexia, respiratory distress, weakness, right-sided cardiac murmur, anemia, hemoglobinuria, hepatic and renal dysfunction, signs of forward and backward heart failure, and, possibly, disseminated intravascular coagulation (DIC). Retrograde migration of adult heartworms from the pulmonary arteries to the right ventricle, right atrium, and venae cavae causes disruption of the tricuspid apparatus. Valvular insufficiency, with concurrent pulmonary hypertension, reduces cardiac output thus resulting in forward and backward heart failure. Additionally, red blood cells are traumatized and hemolyzed as they flow through the mass of worms. Therapy consists of supportive care and the removal of the heartworm mass from the right ventricular inflow tract. Caval syndrome in dogs and cats is associated with high mortality rates and generally has a guarded to poor prognosis.