Breast cancer survivors have increased risks of developing second primary cancers due to shared etiology, life style factors but also to primary breast cancer treatment. Among 53 418 patients registered by the population based Danish Breast Cancer Cooperative Group (DBCG) during 1977-2001, 31 818 patients were treated and followed according to guidelines of DBCG. In addition to surgery 23% received tamoxifen, 23% chemotherapy and 35% radiotherapy as treatment for primary breast cancer. Second primary cancers were identified by linkage to the population based Danish Cancer Register. Cancer incidence rates of the Danish population were used for calculation of standardized incidence ratios (SIRs). Time at risk was from diagnosis of breast cancer+1 year until death or through 2002. Risk for all second primary cancers combined was increased, SIR=1.04 (95% confidence interval 0.99-1.08). Sites with significantly elevated risks included corpus uteri (SIR=1.23), ovary (1.39), soft tissues (2.24), acute leukaemia (2.02), and sites potentially inducible by breast cancer radiotherapy combined (1.11). For irradiated patients compared to non-irradiated the risk was increased for all sited combined, radiotherapy-related sites, colon and soft tissues. Tamoxifen treated had, compared to non-treated, elevated risk for cancer of corpus uteri (SIR=1.83 vs 1.04). Patients given adjuvant chemotherapy had, compared to those not, elevated risks for all sites combined (SIR=1.24 vs 1.01) and for ovary (2.16 vs 1.24). Risk for cancer of the lung, uterus and ovary was analysed using multivariate Poisson regression. For lung cancer the risk was related to radiotherapy and time since diagnosis, the relative risk for lung cancer being 1.33 (95% CI 1.00-1.77) (irradiated vs non-irradiated). Ovary cancer risk was inversely related to age at diagnosis but not to treatment and corpus uteri cancer risk related to tamoxifen treatment, relative risk 1.57. The findings are in accordance to other population based studies.