PURPOSE: Sarcopenia, defined as a decrease in the skeletal muscle mass and its function, is associated with a poor clinical outcome in several malignancies. We aimed to examine whether sarcopenia can be a predictor of incompletion of concurrent chemoradiotherapy (CCRT) and survival for head and neck cancer (HNC) patients. METHODS: Forty-one male HNC patients who received CCRT were enrolled in the study. Cross-sectional muscle areas at the third lumbar vertebral level were normalized by the squared height of the patients and were termed the lumbar skeletal muscle index (LSMI, cm2/m2), a marker of sarcopenia. Patients were divided into high (30/41, 73%) and low (11/41, 27%) LSMI groups. The LSMI cut-off value was set at 39.7 cm2/m2 based on a receiver operating characteristic curve for incompletion of CCRT. The groups were compared for survival rate by the Kaplan-Meier method. Factors predicting incompletion of CCRT were investigated among several variables. RESULTS: Multivariate analysis showed that a pre-treatment low LSMI (P = 0.033) and age over 70 years (P = 0.023) were the only significant predictors for incompletion of CCRT. The 2-year disease-specific survival (DSS) rate was significantly lower in the low LSMI group (61%) than in the high LSMI group (97%, P = 0.012), whereas there were no differences in the DSS rate between the low and high body mass index groups. CONCLUSION: The prevalence of sarcopenia in HNC patients receiving CCRT was 27%. Its presence before treatment was a significant predictor of incomplete CCRT and poor DSS rate in HNC patients.