3080 Background: Immunotherapy with checkpoint blockade was recently approved for patients with recurrent/metastatic SCCHN, however it has not been investigated in the curative-intent setting yet. In this study, we investigated the T-cell receptor repertoire and the immune microenvironment in tumor tissues of SCCHN patients with locoregionally advanced disease. Methods: T-cell receptor sequencing and polymerase chain reaction for immune-related genes of tumor tissues from 44 patients with locoregionally advanced SCCHN prior to treatment with definitive chemoradiotherapy were conducted. T-cell receptor clonality and the mRNA expression levels of immune-related genes were correlated with various clinicopathological parameters. Results: In patients with locoregionally advanced SCCHN, tumor infiltrating T-cells clonally expand and GRZB mRNA levels were associated significantly with longer progression-free survival (PFS) (p = 0.003) independent of HPV status, tumor and nodal stage. The TCR-β DI was significantly lower in HPV-negative compared to HPV-positive tumors (p = 0.002), signifying more clonal T-cell expansion in HPV-negative tumors. A higher percentage of HPV-negative tumors expressed HLA-A protein compared to HPV-positive tumors (p = 0.049), suggesting that the greater T-cell clonal expansion might be due to more robust antigen presentation by HPV-negative tumors. Conclusions: This study suggests the pre-existence of clonally expanded T-cells in patients with locoregionally advanced SCCHN prior to treatment, and provides rationale to introduce immunotherapy in the curative-intent setting. The association of high GRZB mRNA levels with favorable PFS independent of HPV-status, tumor and nodal stage supports that the pre-existence of an intrinsically inflamed microenvironment enhances chemoradiotherapy effects. Finally, in HPV-positive tumors, the T-cell infiltrate seemed to be more diverse which could be secondary to virally-induced defective expression of HLA class I molecules.