The purpose of this study was to assess the role of treatment in long-term overall and cardiovascular mortality after childhood cancer. We studied 4,122 5-year survivors of a childhood cancer diagnosed before 1986 in France and the United Kingdom. Information on chemotherapy was collected, and the radiation dose delivered to the heart was estimated for 2,870 patients who had received radiotherapy. After 86,453 person-years of follow-up (average, 27 years), 603 deaths had occurred. The overall standardized mortality ratio (SMR) was 8.3-fold higher (95% CI, 7.6-fold to 9.0-fold higher) in relation to the general populations in France and the United Kingdom. Thirty-two patients had died as a result of cardiovascular diseases (ie, 5.0-fold 95% CI, 3.3-fold to 6.7-fold more than expected). The risk of dying as a result of cardiac diseases (n = 21) was significantly higher in individuals who had received a cumulative anthracycline dose greater than 360 mg/m(2) (relative risk RR, 4.4; 95% CI, 1.3 to 15.3) and in individuals who received an average radiation dose that exceeded 5 Gy (RR, 12.5 and 25.1 for 5 to 14.9 Gy and > 15 Gy, respectively) to the heart. A linear relationship was found between the average dose of radiation to the heart and the risk of cardiac mortality (estimated excess corrected RR at 1 Gy, 60%). This study is the first, to our knowledge, to establish a relationship between the radiation dose received by the heart during radiotherapy for a childhood cancer and long-term cardiac mortality. This study also confirms a significant excess risk of cardiac mortality associated with a high cumulative dose of anthracyclines.