The aim of this study was to determine a functional cut-off point for trismus in head and neck oncology. In total, 89 patients (13 dentate, 30 partially dentate and 46 edentulous) treated for cancer of the oral cavity or oropharynx were asked whether they experienced a limited mouth opening. The mandibular function impairment questionnaire (MFIQ) was filled out and mouth opening was measured. The proportion correctly predicted (proportion true positives+proportion true negatives) was calculated. For cut-off points from 25 to 45 mm, differences in MFIQ scores of the restricted and non-restricted groups were analyzed. A receiver operating curve was constructed. The proportion correctly predicted was highest for cut-off point <or=35 mm (0.81), with a sensitivity of 0.71 and a specificity of 0.98. Differences in perceived mandibular function impairments were significant for all cut-off points of 35 mm or less. For the total group, the area under curve was 0.87. For the subgroups of dentate, partially dentate and edentulous patients, no consistent cut-off points could be found on the basis of the proportion correctly predicted and the significance of the difference in MFIQ scores. A mouth opening of <or=35 mm is a functional cut-off point for trismus in head and neck oncology patients.