Purpose <p>Nutritional decline is a common challenge in head and neck cancer (HNC) patients undergoing concurrent chemoradiotherapy (CCRT). This study aimed to evaluate weekly trends in body weight and nutritional status to identify the timing and progression of malnutrition risk during treatment, thereby informing the timing of potential nutritional interventions.</p> Methods <p>This observational longitudinal study enrolled 23 newly diagnosed HNC patients (87% male, median age 56.7&#xa0;years) receiving radiotherapy, with or without concurrent chemotherapy. Weekly weight measurements and Mini-Nutritional Assessment (MNA) scores were recorded throughout the 7-week treatment period. Nutritional counseling and general dietary advice were routinely provided as part of standard clinical care.</p> Results <p>Significant weight loss began in the 5th week of treatment, with an average total weight loss of 5.9% by the end of CCRT. MNA scores showed a clear decline from week 4 onward, and by week 7, most patients had entered the malnourished category. In parallel, 76% of patients required soft or liquid diets by the end of treatment, and the use of oral nutritional supplements increased from 10 to 96%.</p> Conclusion <p>The results demonstrate that the risk of malnutrition increases rapidly during CCRT, particularly after the 4th week of treatment. Regular nutritional monitoring may be essential to detect early deterioration and guide the timing of nutritional support. Further studies are needed to evaluate the efficacy of structured nutritional interventions initiated early in the treatment course.</p>