BACKGROUND: Excess risks of several second neoplasms following breast cancer have been reported. However, these risks have still to be quantified. PATIENTS AND METHODS: We considered 9,729 breast cancer patients registered by the Swiss Cancer Registries of Vaud and Neuchatel (covering about 786,000 inhabitants) and followed up from 1974 to 1998. RESULTS: Overall, 443 second primary neoplasms (other than second primary breast cancers) were observed versus 389 expected [standardised incidence ratio (SIR): 1.14; 95% confidence interval (CI) 1.04-1.25]. The SIRs were above unity for endometrium (SIR = 1.5), ovary (1.3), colorectum (1.1), gallbladder (1.4), cutaneous malignant melanoma (1.4), kidney (1.4), lymphomas (1.4) and leukaemias (1.2), as well as for selected tobacco-related neoplasms. The largest excess risk was found for soft tissue sarcomas (STS) with 10 cases observed versus 3.1 expected (SIR = 3.2; 95% CI 1.5-5.9). Of these, eight occurred in potentially irradiated areas. CONCLUSIONS: This analysis confirms the existence of a modest excess in several neoplasms occurring after breast cancer. The substantial excess of STS confirms the strong association between irradiation and STS. [authors]