Several cut-off points for trismus in head and neck cancer patients have been used. A mouth opening of 35 mm or less is most frequently used as cut-off point. Due to the variation in cut-off points, prevalence, risk factors and treatment outcomes of trismus cannot be studied in a uniform manner. To provide uniformity, we aimed to verify the cut-off point of 35 mm or less. Additionally, we aimed to determine associated covariates with reported difficulties when opening the mouth.