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Exposure of the breast to ionising radiation increases the risk of breast cancer, especially among young women. However, some issues remain controversial, for instance the shape of the dose-response curve and the expression of time-related excess. The main purpose of this report was to examine the dose-response curves for radiation-induced breast cancer formulated according to radiobiological target theories. Another purpose was to analyse the time-related excess of breast cancer risk after exposure when dose and age at first exposure were held constant. Breast cancer incidence was analysed in a cohort of 3090 women diagnosed with benign breast disease during 1925-61 (median age 37 years). Of these, 1216 were treated with radiation therapy. The dose range was 0-50 Gy (mean 5.8 Gy). The incidence rate as function of dose was analysed using a linear-quadratic Poisson regression model. Cell-killing effects and other modifying effects were incorporated through additional log-linear terms. Additive and multiplicative models were compared in estimating the time-related excess. The analysis, which was based on 278 breast cancer cases, showed a linear dose-response relationship at low to medium dose levels with a cell-killing effect of 5% Gy-1 (95% confidence interval 2-9%). For a given absorbed dose and age at first exposure the time-related excess was proportional to the background rates with a suggestion that the excess remains throughout life.