BACKGROUND Head and neck cancers (HNC) are the 7th most prevalent neoplasms in the world. In 50% of these patients, body weight loss and malnutrition are observed before the beginning of therapy. It is known that an important role in the pathomechanism of malnutrition and cachexia is played by the development of inflammation, degradation of muscle fibers, and browning of white adipose tissue (WAT). It was demonstrated that even a slight increase in irisin concentration leads to browning of WAT. MATERIAL AND METHODS The study group consisted of 50 patients with HNC. The nutritional status of the patients was assessed by the Nutritional Risk Score 2002 (NRS 2002) and Subjective Global Assessment (SGA) scales. Using bioelectrical impedance analysis (BIA), the parameters fat mass (FM) and fat-free mass (FFM) were obtained. RESULTS Higher irisin values (1.57 vs 1.18 [ng/ml], P=0.0004) were observed in patients with higher nutritional risk (≥3) evaluated according to the NRS scale. In patients assessed as B or C on the SGA scale, higher values of irisin concentration (1.38 vs 1.07 [ng/ml], P=0.0139) were noted. It was also observed that the level of irisin before treatment was negatively correlated (rho=-0.30, p=0.0350) with FM% and was positively correlated (rho=0.30, p=0.0340) with FFM% in BIA measurements performed after the 7th cycle of RTH. CONCLUSIONS Based on these results, we conclude that patients with malnutrition tend to have higher irisin values compared to normally nourished patients. A high level of irisin may be a useful marker of malnutrition in patients with HNC.