Multiple primary malignancies in patients with malignant tumours of the nasal cavities and paranasal sinuses. The incidence of multiple primary malignancies (MPM) was analysed in 1297 patients with malignant tumours of the nasal cavities and paranasal sinuses. The patients were followed for a minimum of six years or until death. The incidence of MPM was 3.4 per cent (44/1297) which is lower in patients with malignant tumours of other regions in the upper aerodigestive tract. Five of the 44 patients had second malignancies within previously irradiated areas. There was a difference between the proportion of histologies occurring in irradiated tissues compared with the overall 1297 patients. This finding suggests that radiotherapy may induce a second cancer. 