Two patients who developed frank arterial bleeding after combined microwave-induced hyperthermia and radiotherapy are described. One patient received re-irradiation and hyperthermia for recurrent metastatic neck nodes of a mesopharyngeal carcinoma. Full course radiotherapy had been given 6 years previously and a right-sided radical neck node dissection had been performed 4 months earlier because of recurrent neck node metastases. Six weeks after the combined therapy for a second recurrence, which achieved complete remission, a fatal rupture of the carotid artery occurred. The other patient received re-irradiation and hyperthermia for a chest wall recurrence of a breast carcinoma, treated 5.5 years previously by sector resection and tangential beam radiotherapy, and treated again 2 years earlier with extensive surgery for a local recurrence. A frank arterial bleeding from the treated region was seen after 7 months, but could be arrested with surgery. This important complication in combined hyperthermia and radiotherapy does not seem to have been recognized before. Different explanations are discussed, such as the previous local treatment as well as high temperature and atherosclerosis per se.