Radiotherapy is commonly used for treatment of malignant disease. As a consequence of radiotherapy, an increased risk of developing a second malignant neoplasm has been shown. However, little is known about the effects of radiation on developing sarcoma. The aim of this study was to examine the risk of developing a bone or soft tissue sarcoma after radiotherapy for a first primary cancer. The study population included all the patients with primary cancers of breast, cervix uteri, corpus uteri, lung, ovary, prostate, rectum and lymphoma diagnosed during 1953–2000 and identified from the Finnish Cancer Registry. Patients were followed up for subsequent sarcomas. The follow-up yielded 1.5 million person-years at risk and 147 sarcomas. Compared to the national incidence rates, after 10 years of follow-up sarcoma risk was increased among patients who had received neither radiotherapy nor chemotherapy (standardised incidence ratio (SIR) 2.0, 95% CI 1.3–3.0), radiotherapy without chemotherapy (SIR 3.2, 95% CI 2.3–4.3), chemotherapy without radiotherapy (SIR 4.9, 95% CI 1.0–14.4), as well as combined radiotherapy and chemotherapy (SIR 3.4, 95% CI 0.4–12.5). For radiotherapy in ages below 55 the SIR was 4.2 (95% CI 2.9–5.8). In the adjusted regression analysis the rate ratio was 1.5 (95% CI 0.9–2.6) for the radiotherapy group. In conclusion, radiotherapy appears to be associated with an increased risk of developing sarcoma especially among younger patients. Further investigation is needed to clarify the dose–response of the preceding ionizing radiation. © 2005 Wiley-Liss, Inc.