Background-The aim of this study was to investigate the incidence, histopathology, demographics, and survival associated with primary malignant cardiac tumors (PMCTs). Methods and Results-We queried the Surveillance, Epidemiology and End Results (SEER) 18 registry from the National Cancer Institute for all PMCTs diagnosed from 1973 to 2011. We describe PMCT histopathology and incidence, comparing characteristics and survival of these patients with those of patients with extracardiac malignancies of similar histopathology. From a total of 7 384 580 cases of cancer registered in SEER, we identified 551 PMCTs (0.008%). The incidence of PMCT diagnosis is 34 cases per 100 million persons and has increased over time (25.1 in 1973-1989, 30.2 in 1990-1999, and 46.6 in 2000-2011). Most patients are female (54.1%) and white (78.6%) with median age at diagnosis of 50 years. The most common PMCTs are sarcomas (n=357, 64.8%), followed by lymphomas (n=150, 27%) and mesotheliomas (n=44, 8%). Most patients are diagnosed with tissue sample (96.8%). Although use of chemotherapy is not documented in SEER, 19% of patients received radiation and 44% had surgery. After a median follow-up of 80 months, 413 patients had died. The 1-, 3-, and 5-year survival rates were 46%, 22%, and 17% and have improved over the eras, with 1-, 3-, and 5-year survival rates of 32%, 17%, and 14% for 1973 to 1989 and 50%, 24%, and 19% for 2000 to 2011 (P=0.009). Cardiac sarcomas and mesotheliomas are the most lethal PMCTs, with 1-, 3-, 5-year survival rates of 47%, 16%, and 11% and of 51%, 26%, and 23% compared with 59%, 41%, and 34% for lymphomas, respectively (log rank test P<0.001). Patients with cardiac lymphomas and sarcomas are younger and have worse survival than patients with extracardiac disease of similar histopathology (P<0.001). Conclusions-PMCTs are extremely rare and continue to be associated with poor prognosis. Over the past 5 decades, the incidence and survival of patients diagnosed with PMCT appear to have increased.