In order to elucidate the causal mechanism of trismus in head and neck cancer, 21 patients manifesting trismus out of 212 patients with malignant tumours in the head and neck (treated in Tokyo University Branch Hospital from 1983 to 1991) were reviewed. Nine patients developed trismus either by infiltration of the muscles of mastication or by reflex spasm. Trismus was considered to have developed as a result of irradiation in five cases and of surgical intervention in seven cases. In some cases of oropharyngeal cancer, CT revealed no evidence of tumour invasion into the infratemporal fossa when trismus occurred, suggesting that trismus was caused either by the reflex spasm of muscles or by microinvasion too small to be seen in CT films. Maxillary sinus tumours were often without trismus even when they extended posteriorly to the infratemporal fossa.