Among women with breast cancer, we compared the relative and absolute rates of subsequent cancers in 1541 women treated with radiotherapy (RT) to 4570 women not so treated (NRT), using all registered in the Swiss Vaud Cancer Registry in the period between 1978 and 1998, and followed up to December 2002. Standardised incidence ratios (SIRs) and the corresponding 95% confidence intervals (CIs) were based on age- and calendar year-specific incidence rates in the Vaud general population. There were 11 lung cancers in RT (SIR=1.40; 95% CI: 0.70-2.51) and 17 in NRT women (SIR=0.76; 95% CI: 0.44-1.22), 72 contralateral breast cancers in RT (SIR=1.85; 95% CI: 1.45-2.33) and 150 in NRT women (SIR=1.38; 95% CI: 1.16-1.61), and 90 other neoplasms in RT (SIR=1.37; 95% CI: 1.10-1.68) and 224 in NRT women (SIR=1.05; 95% CI: 0.91-1.19). Overall, there were 173 second neoplasms in RT women (SIR=1.54, 95% CI: 1.32-1.78) and 391 among NRT women (SIR=1.13, 95% CI: 1.02-1.25). The estimates were significantly heterogeneous. After 15 years, 20% of RT cases vs 16% of NRT cases had developed a second neoplasm. The appreciable excess risk of subsequent neoplasms after RT for breast cancer must be weighed against the approximately 5% reduction of breast cancer mortality at 15 years after RT. [Authors]