Idiopathic hemorrhagic pericardial effusion was diagnosed in 8 dogs. The patients were typically males of large or giant breeds and a wide age range was represented. In all dogs clinical features of acute or chronic cardiac tamponade and right-sided heart failure were present. The pericardial effusion in each case was identified by thoracic radiography, electrocardiography, and echocardiography. Cytologic examination of the fluid did not allow differentiation from hemorrhagic effusions caused by neoplasia. Bacterial and fungal cultures were negative in 5 dogs. In 6 cases, a presumptive diagnosis was based on the absence of cardiac masses on 2-dimensional echocardiography, contrast pericardiography, or both. The condition was managed successfully by partial pericardiectomy in 5 cases. The definitive diagnosis in each case was established by gross cardiac examination at surgery or necropsy and by histologic examination of tissues. Blood vessels and lymphatics of the parietal and visceral pericardia appeared to be the primary targets of the disease process.