Among 152 600 breast cancer patients diagnosed during 1958-2000, there was a 22% increased risk of developing a second primary non-breast malignancy (standardised incidence ratio (SIR)=1.22; 95% confidence interval (CI): 1.19-1.24). The highest risk was seen for connective tissue cancer (SIR=1.78; 95% CI: 1.49-2.10). Increased risks were noted among women diagnosed with breast cancer before age 50. Oesophagus cancer and non-Hodgkin's lymphoma showed six- and four-fold higher risks, respectively, in women with a family history of breast cancer compared to those without in the > or =10-year follow-up period.