Objective: The objective of this study is to assess cause-specific mortality for patients with breast cancer and to determine if excess cardiac death was associated with radiation therapy (RT). Methods: We obtained 10-year cause-specific mortality information from the New South Wales (NSW) Central Cancer Registry and National Death Index on 1242 patients with unilateral stage I-III invasive breast cancer in NSW, Australia, diagnosed over a 6-month period in 1995. We compared actuarial cause-specific mortality (breast cancer, cardiac, other cancers and other causes) for patients who received left-sided, right-sided or no RT. Results: Mortality due to breast cancer or due to other cancers was not significantly different (P = 0.30 and P = 0.11) between the three subgroups. Mortality due to cardiac and other causes was higher in patients who did not have radiotherapy (P = 0.001 and P < 0.001). A total of 52 cardiac deaths in 1242 patients (4.2%) occurred - six of 274 patients (2.2%) in the left-sided radiotherapy group, four of 245 patients (1.6%) in the right-sided radiotherapy group (P = 0.63) and 42 of 723 patients (5.8%) in the no radiotherapy group. Most cardiac deaths (46 of 52 cases) occurred in patients aged 70 years or older at the time of diagnosis. There were no differences in cardiac mortality between the three treatment groups for those aged 70 years or older (P = 0.22, log-rank test), suggesting that the higher overall cardiac mortality rate in the no-RT group is due to a higher percentage of patients aged 70 years or older. Of the 10 patients who died from cardiac causes and who had received RT, none had received chemotherapy or irradiation to the internal mammary chain. Conclusion: There is no excess cardiac mortality due to RT within the first decade in a population series of patients with breast cancer treated with modern radiotherapy.