Radiotherapy, when used in head and neck cancer treatment, can produce side effects in the patients, such as decreased salivary production, xerostomia, opportunistic infections, radiation caries, dysphagia, local discomfort and the limitation of mouth opening. The aim of this study was to evaluate the amplitude of mouth opening in patients before and immediately after the completion of radiotherapy, comparing the effectiveness of two physiotherapy exercises. The irradiated sites included the masticatory muscles. The results demonstrated that there were no statistically significant differences between the two instituted exercises; however there was a trend towards better clinical results in group 2. The amplitude of mouth opening showed a trend towards reduction, but this was not statistically significant. When the pterygoid and sternocleidomatoid muscles were included in the irriated field, patients were observed to have more morbidity. This indicates the great importance of these muscles in mouth opening. Based on the results obtained within this study, it is not possible to conclude that physiotherapy exercies are efficacious in preventing trismus. Future longitudinal studies are required to verify the onset of trismus in radiotherapy patients.