Purpose: To study the risk of ischemic stroke (IS) following carotid (CAS) or vertebral artery stenosis (VAS) in head and neck cancer (HNC) patients after radiation therapy (RT). Methods: We included HNC patients who received RT between 2010 and 2023. They underwent regular head and neck imaging monitoring to evaluate cancer recurrence at the department of radiation-oncology and vascular examinations at the department of neurology. Patients were initially divided into nasopharyngeal carcinoma (NPC) and non-NPC groups. The primary outcome was the occurrence of IS after RT, and the secondary outcomes included the development of >50% CAS or >50% VAS after RT. Cox regression and Kaplan?Meier analyses were conducted to compare the outcomes of the study groups. Further analysis was conducted based on the presence or absence of >50% CAS or >50% VAS during the follow-up period. Results: Of the 1,423 HNC patients, 19%, 6.8%, and 2.3% developed >50% CAS, >50% VAS, and IS during a 58-month follow-up. Compared with the NPC group, the non-NPC group exhibited a higher incidence of >50% CAS (25.4% vs. 10.7%, p<0.001) and >50% VAS (8.8% vs. 4.3%, p<0.001), but similar risks of IS. In patients with HNC, >50% CAS (adjusted hazard ratio [HR]=3.21, 95% confidence interval [CI]=1.53-6.71), and >50% VAS (adjusted HR=2.89, 95% CI=1.28-6.53) were both the independent predictors of IS. In the patients with NPC, >50% CAS was an independent predictor of anterior circulation IS (adjusted HR=4.39, 95% CI=1.17-16.47). By contrast, >50% VAS emerged as a predictor of posterior circulation IS in both the NPC (adjusted HR=15.02, 95% CI=3.76-60.06) and non-NPC groups (adjusted HR=13.59, 95% CI=2.21-83.46). Conclusion: HNC patients with >50% CAS or >50% VAS after RT had an increased risk of IS within their corresponding vascular territory. CAS is a major predictor of IS in NPC patients, whereas VAS is a major predictor of IS in both NPC and non-NPC patients.