During the period 1975 to 1984, a histopathologic diagnosis of primary cardiac hemangiosarcoma was made in 38 dogs at Angell Memorial Animal Hospital. The diagnosis was confirmed by exploratory thoracotomy in 16 cases and at necropsy in 22 cases. At the time of exploratory thoracotomy, 7 dogs were euthanatized because of nonresectability of the primary tumor and/or gross metastatic disease. In 9 dogs, the tumor was resected by removing part of the right atrium. Complications included atrial and ventricular arrhythmias, anemia, disseminated intravascular coagulation, and pneumonia. Prolonged and multiple hospitalizations were a common feature of the postoperative period. Adjuvant therapy was not utilized in any case. The mean survival time was 4 months (2 days to 8 months).