Background: The whole neck is often included in the radiation therapy (RT) field of head and neck cancer (HNC), and carotid vessels of the neck may be affected by radiation in a manner that increases the risk of ischemic stroke (IS). In the present study, we analyzed the relationship between RT for HNC and subsequent IS using a database managed by the Health Insurance Review & Assessment Service. Materials and Methods: A public insurance system was used to identify a nationwide cohort of patients diagnosed with HNC that underwent RT. The incidence of IS after RT was determined up to the end of December 2018, and associated risks were analyzed. In addition, patients that underwent frequent elective neck irradiation as determined by disease classification or less frequent elective neck irradiation were allocated to groups A and B, respectively. Results: 12,079 HNC patients that received RT between 2010 and 2014 were identified. 472 (3.9%) of the subjects experienced IS and incidence rates were > 5% for hypopharyngeal cancer and supraglottic cancer. The incidence of IS was 5.4% in patients > 60 years and 2.2% in patients < 60 years (relative risk (RR) 2.59). In addition, the incidence of IS was greater in group A than in group B (RR 1.29), and in men than in women (RR 1.33). Conclusion: When performing extensive RT involving the whole neck in older men, close follow-up and prophylactic medication should be considered to prevent IS.