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Background and purpose: Doses to the coronary arteries in breast cancer (BC) radiotherapy (RT) have been suggested to be a risk predictor of long-term cardiac toxicity after BC treatment. We investigated the dose–risk relationships between near maximum doses (Dmax) to the right coronary artery (RCA) and left anterior descending coronary artery (LAD) and ischemic heart disease (IHD) mortality after BC RT. Patients and methods: In a cohort of 2,813 women diagnosed with BC between 1958 and 1992 with a follow-up of at least 10 years, we identified 134 cases of death due to IHD 10–19 years after BC diagnosis. For each case, one control was selected within the cohort matched for age at diagnosis. 3D-volume and 3D-dose reconstructions were obtained from individual RT charts. We estimated the Dmax to the RCA and the LAD and the mean heart dose (MHD). We performed conditional logistic regression analysis comparing piecewise spline transformation and simple linear modeling for best fit. Results: There was a linear dose–risk relationship for both the Dmax to the RCA (odds ratio [OR]/Gray [Gy] 1.03 [1.01–1.05]) and the LAD (OR/Gy 1.04 [1.02–1.06]) in a multivariable model. For MHD there was a linear dose–risk relationship (1,14 OR/Gy [1.08–1.19]. For all relationships, simple linear modelling was superior to spline transformations. Interpretation: Doses to both the RCA and LAD are independent risk predictors of long-term cardiotoxicity after RT for BC In addition to the LAD, the RCA should be regarded as an organ at risk in RT planning.