Background: During a 6-week course of (chemo)radiation many head and neck cancer patients have to endure radiotherapy-induced toxicity, negatively affecting patients' quality of life. Pretreatment counseling combined with self-help exercises could be provided to inform patients and possibly prevent them from having speech, swallowing, and shoulder problems during and after treatment. Objective: Our goal was to investigate the feasibility of a multimodal guided self-help exercise program entitled Head Matters during (chemo)radiation in head and neck cancer patients. Methods: Head and neck cancer patients treated with primary (chemo)radiation or after surgery were asked to perform Head Matters at home. This prophylactic exercise program, offered in three different formats, aims to reduce the risk of developing speech, swallowing, shoulder problems, and a stiff neck. Weekly coaching was provided by a speech and swallowing therapist. Patients filled out a diary to keep track of their exercise activity. To gain insight into possible barriers and facilitators to exercise adherence, reports of weekly coaching sessions were analyzed by 2 coders independently. Results: Of 41 eligible patients, 34 patients were willing to participate (83% uptake). Of participating patients, 21 patients completed the program (64% adherence rate). The majority of participants (58%) had a moderate to high level of exercise performance. Exercise performance level was not significantly associated with age (P=.50), gender (P=.42), tumor subsite (P=1.00) or tumor stage (P=.20), treatment modality (P=.72), or Head Matters format (Web-based or paper) (P=1.00). Based on patients' diaries and weekly coaching sessions, patients' perceived barriers to exercise were a decreased physical condition, treatment-related barriers, emotional problems, lack of motivation, social barriers, and technical problems. Patients' perceived facilitators included an increased physical condition, feeling motivated, and social and technical facilitators. Conclusions: Head Matters, a multimodal guided self-help exercise program is feasible for head and neck cancer patients undergoing (chemo)radiation. Several barriers (decreased physical condition, treatment-related barriers) and facilitators (increased physical condition, feeling motivated) were identified providing directions for future studies. The next step is conducting a study investigating the (cost-)effectiveness of Head Matters on speech, swallowing, shoulder function, and quality of life. © Holly O Witteman.