BACKGROUND: Radiotherapy (RT) in head and neck cancer (HNC) can cause multiple side effects such as nausea, pain, taste loss, fatigue, oral mucositis, xerostomia, and acute radiation-associated dysphagia (RAD). These factors threaten patients' oral intake (OI) during this RT. Reduced OI can cause weight loss, dehydration, malnutrition, and various comorbidities. On top, reduced OI significantly affects quality of life and may contribute to RAD through the disuse of swallowing muscles. With the aim of maximizing the retention of a patient's OI, it is important to gain an insight into the factors that have the greatest impact. Therefore, this study aims to identify the impact of contributing factors on decreased OI during RT. METHODS: During their treatment, 55 HNC patients completed an OI questionnaire at 5 different time points: during weeks 1, 2, 3, and 4 and at the end of RT (week 7). First, patients rated the OI compared to pre-RT on a 100 mm visual analogue scale (VAS). Subsequently, patients reported on separate VAS the degree to which pain, fatigue, loss of taste, loss of smell, loss of interest in food, nausea, and loss of hunger contributed to the decrease in OI (0: no contribution; 100: complete contribution). SPSS version 27 was used to analyze the results. RESULTS: OI decreased over time during RT, with the lowest OI at the end of RT. During the first 4 weeks of RT, the impact of all factors with pain, loss of taste, loss of interest in food, and loss of hunger pointed out as strongest contributing factors to a decreased OI. The most important patient-reported impacting factor on OI was loss of taste. At the end of RT, the importance of pain and nausea still increases, while the contribution of the other factors drops slightly. CONCLUSION: This cohort study shows that several factors contribute to a decreased OI in HNC patients during RT. This study is the first prospective analysis to identify self-reported factors contributing to reduced OI. Results demonstrate that taste has the greatest impact on OI followed by loss of interest in food, loss of hunger, and pain.