The aim of the study was to examine the following: (a) the trajectory of external and internal head and neck lymphoedema (HNL) in patients with head and neck cancer (HNC) up to 12 months post-chemoradiotherapy (CRT) and (b) the relationship between HNL and swallowing function. Using a prospective longitudinal cohort study, external/internal HNL and swallowing were examined in 33 participants at 3, 6 and 12 months post-CRT. External HNL was assessed using the Assessment of Lymphoedema of the Head and Neck and the MD Anderson Cancer Centre Lymphoedema Rating Scale. Internal HNL was rated using Patterson’s Radiotherapy Oedema Rating Scale. Swallowing was assessed via clinical, instrumental and patient-reported measures. Associations between HNL and swallowing were examined using multivariable regression models. External HNL was prevalent at 3 months (71%), improved by 6 months (58%) and largely resolved by 12 months (10%). In contrast, moderate/severe internal HNL was prevalent at 3 months (96%), 6 months (84%) and at 12 months (65%). More severe penetration/aspiration and increased diet modification were associated with higher severities of external HNL (p=0.006 andp=0.031, respectively) and internal HNL (p<0.001 andp=0.007, respectively), and more diffuse internal HNL (p=0.043 andp=0.001, respectively). Worse patient-reported swallowing outcomes were associated with a higher severity of external HNL (p=0.001) and more diffuse internal HNL (p=0.002). External HNL largely resolves by 12 months post-CRT, but internal HNL persists. Patients with a higher severity of external and/or internal HNL and those with more diffuse internal HNL can be expected to have more severe dysphagia.