Objectives: To compare the impact on survival and late radiation toxicity of patients with squamous cell carcinoma of unknown primary who were treated with ipsilateral neck dissection and ipsilateral postoperative radiotherapy (PORT) and patients treated with ipsilateral neck dissection and PORT to both sides of the neck plus the pharyngeal axis. Methods: Retrospective cohort study performed at the Erasmus University Medical Center in which 78 patients with squamous cell carcinoma of unknown primary were identified. Thirty-nine patients received PORT to both sides of the neck plus the pharyngeal axis (BILAX) and another 39 patients were irradiated on the ipsilateral neck (IPSI) only. The endpoints of the present study were 3-year overall survival (OS), 3-year disease-free survival (DFS), and overall late radiation toxicity. Results: The 3-year OS rate of the entire group of 78 patients was 74.2% and the 3-year DFS rate was 72.7%. The 3-year OS rates for the IPSI and the BILAX groups were 74.4% and 74.1%, respectively (P =.654). The most common late radiation toxicity experienced was xerostomia (64.8%), which was significantly more often seen in the BILAX group than in the IPSI group (83.8% and 44.1%, respectively, P =.001). Overall late radiation toxicity was significantly higher in the BILAX group (P =.003). Conclusion: There was no significant difference in OS and DFS rates between the IPSI and the BILAX group. Late radiation toxicity was significantly higher in the BILAX group. Level of Evidence: Level 2b: Individual retrospective cohort study.