To determine the usefulness of prophylactic radiotherapy following thoracoscopy or pleural aspiration in patients with mesothelioma we reviewed the case notes of patients treated at St Mary's Hospital between 1990 and 1994. Twenty patients have received treatment to 38 sites and tumour seeding was prevented in all patients available for follow-up. Four patients acted as their own controls by developing nodules at untreated sites. Prophylactic radiotherapy is highly effective in preventing tumour seeding in mesothelioma.