BACKGROUND AND PURPOSE: To assess the effectiveness of radiotherapy in preventing tumour seeding after chest drain or pleural biopsy in patients with malignant mesothelioma and to determine, if tract metastases appear, whether they are tender or troublesome to patients. PATIENTS AND METHODS: Patients with a histological diagnosis of pleural mesothelioma and an invasive procedure within the preceding 21 days were stratified by age, performance status and treatment centre. Randomisation was performed between immediate drain site radiotherapy 21Gy in three fractions (XRT arm) or best supportive care (BSC) with follow-up to 12 months. Patients were asked to complete questionnaires on treatment toxicity and on symptoms from any tract metastases detected. RESULTS: Sixty-one patients were recruited from two centres between 1998 and 2004; 56 men, 5 women, median age 70. 31 were allocated to drain site radiotherapy. Seven patients developed tract metastases associated with the drain site (four XRT arm, three BSC) and four developed metastases associated with subsequent procedures at other sites (three XRT, one BSC). Two patients each developed two tract metastases. Of the 12 metastases, nine overlay the previous drain site but three were adjacent to the site. No statistically significant difference was found in the risk of tract metastasis associated with the drain site between the arms (p=0.748). CONCLUSIONS: Prophylactic drain site radiotherapy in malignant pleural mesothelioma does not reduce the incidence of tumour seeding by the margin indicated by previous studies.