PURPOSE: To retrospectively assess the prognostic meaning of tumor characteristics depicted on pretreatment magnetic resonance (MR) images for local outcome in supraglottic squamous cell carcinoma treated with definitive radiation therapy. MATERIALS AND METHODS: Pretreatment MR images acquired in 84 patients with supraglottic carcinoma treated with curative radiation therapy were reviewed for tumor involvement of laryngeal sites including glottis, subglottis, pre-epiglottic space, laryngeal cartilages, and hypopharynx, and for extralaryngeal extension. The volume of each tumor was estimated, and mean tumor volume was calculated for the group of tumors in each T staging category. RESULTS: Results of univariate analysis showed MR imaging-determined primary tumor volume (P =.03), involvement of pre-epiglottic space (P =.008), abnormal signal intensity in thyroid cartilage (P =.04), and extralaryngeal extension beyond thyroid and/or cricoid cartilage (P =.02) to be significant predictors of local control rate. Results of multivariate analysis with the Cox regression model confirmed statistical significance for invasion of pre-epiglottic space (P =.004) and for abnormal signal intensities in thyroid cartilage adjacent to the anterior commissure (P =.04) and in cricoid cartilage (P =.01). Five-year local control rates were calculated from the regression coefficients of three independent MR imaging prognostic factors, and three prognostic groups were identified on the basis of these control rates. The 5-year local control rate in the high-risk group was 35%, significantly lower than the rates in the intermediate- and low-risk groups (60% and 89%, respectively; P =.002). CONCLUSION: MR imaging-determined pre-epiglottic space involvement and abnormal signal intensities in the thyroid cartilage adjacent to the anterior commissure and/or the cricoid cartilage are strong predictors of local outcome in supraglottic carcinoma treated with definitive radiation therapy.