Background. To investigate the dose-volume factors in mastication muscles that are implicated as possible causes of trismus in patients following treatment with intensity-modulated radiotherapy (IMRT) and concurrent chemotherapy for head and neck cancers.Material and methods. All evaluable patients treated at our institution between January 2004 and April 2009 with chemotherapy and IMRT for squamous cell cancers of the oropharynx, nasopharynx, hypopharynx or larynx were included in this analysis (N = 421). Trismus was assessed using CTCAE 4.0. Bi-lateral masseter, temporalis, lateral pterygoid and medial pterygoid muscles were delineated on axial computed tomography (CT) treatment planning images, and dose-volume parameters were extracted to investigate univariate and multimetric correlations.Results. Forty-six patients (10.9%) were observed to have chronic trismus of grade 1 or greater. From analysis of baseline patient characteristics, toxicity correlated with primary site and patient age. From dose-volume analysis, the steepest dose thresholds and highest correlations were seen for mean dose to ipsilateral masseter (Spearman's rank correlation coefficient Rs = 0.25) and medial pterygoid (Rs = 0.23) muscles. Lyman-Kutcher-Burman modeling showed highest correlations for the same muscles. The best correlation for multimetric logistic regression modeling was withV68Gy to the ipsilateral medial pterygoid (Rs = 0.29).Conclusion. Chemoradiation-induced trismus remains a problem particularly for patients with oropharyngeal carcinoma. Strong dose-volume correlations support the hypothesis that limiting dose to the ipsilateral masseter muscle and, in particular, the medial pterygoid muscle may reduce the likelihood of trismus.