Background: Involuntary body weight loss in head and neck cancer is common. Fundamental for weight loss is an energy imbalance where total energy expenditure exceeds energy intake.Aim: To map energy intake and parameters of energy expenditure at the start of and after radiotherapy, and their relation to weight change, body mass index, and immune markers in patients with head and neck cancer.Materials and Methods: Data from 20 patients on energy intake (24-hour dietary intake recalls), total energy expenditure (SenseWear Armband Pro3), resting energy expenditure (indirect calorimetry), body weight, body mass index, and immune markers in serum (C-reactive protein and Interleukin-6) were collected at the start of and after radiotherapy (median 8 mo, range 5-13).Results: No statistical significance was shown between the two measurement points for energy intake or for the different parameters of energy expenditure. Median values for energy balance were 0.93 and 0.96 for the start of treatment and follow-up, respectively. Twelve and 13 patients had a negative energy balance at the start of radiotherapy and at follow-up, respectively.Conclusion: A negative energy balance was seen for the majority of patients, which stresses the importance of nutritional treatment at the start of and after radiotherapy.