6001 Background: This is a retrospective analysis of the phase III IMCL-9815 trial assessing the role of HPV status in LASCCHN patients (pts) receiving RT+cet or RT by measuring p16 status. Methods: The intent-to-treat (ITT) population (424 pts) was randomized to RT+cet or RT. p16INK4A status was determined by immunohistochemistry. Results: 312/424 (74%) of the ITT pts were p16 evaluable. Baseline characteristics in the ITT and p16 evaluable pts were similar. In both p16+ and p16- pts, the addition of cet to RT improved locoregional control (LRC), overall survival (OS), and progression-free survival (PFS) (Table). Univariate analyses showed a more pronounced treatment effect of RT+cet vs RT in p16+ pts across all endpoints in both the ITT and the oropharyngeal (OPC) populations; however, interaction tests for LRC, OS and PFS (ITT and OPC) did not demonstrate a significant interaction between p16 status and treatment effect. There were no new safety findings. Conclusions: p16 tumor status is strongly progn...