Background: Pts with regionally advanced melanoma were treated with neoadjuvant ipi and HDI in a reported study (Tarhini.J Clin Oncol suppl, 2016; abstr 9585). Pathologic complete response (pCR) was found in 32% of pts. Clonality of T cell repertoire was investigated in TME and peripheral blood mononuclear cells (PBMC). Methods: Pts were randomized to neoadjuvant ipi 3 or 10 mg/kg combined with HDI. Tumor biopsies were evaluable for testing at pretreatment (N = 20 pts) and definitive surgery (week 6-8; N = 25). When available, primary (N = 24) and relapse tumors (N = 6) were tested. PBMC: pretreatment (N = 29), 6 weeks (wk) (N = 24), then 3 (N = 23), 6 (N = 21), 12 (N = 14) months. T cell receptor beta chain (TCRB) repertoire was immunosequenced in PBMC and TME to determine repertoire clonality and T cell fraction in blood and TME (TIL; fraction of all nucleated cells identified as T cells). Results: PBMC T cell fraction when measured early on-treatment (6 wks) was significantly higher in pts who had pCR or microscopic residual disease vs. gross disease at the 6-8 wks surgery (p = 0.047). PBMC clonality was significantly lower at 12 wks (p = 0.025) for pts who continued to be relapse free (NED) long term vs. those who eventually relapsed. In TME, except for trends no significant difference in clonality was seen, but in pts with pCR TIL fraction was significantly higher when measured in primary tumors (p = 0.033). The number of tumor-associated clones that were expanded in blood post-treatment was strongly correlated with both TIL fraction (Rho 0.7299, p = 0.0003) and TIL clone diversity (Rho 0.882, p = 2.7^-7). Conclusions: Higher T cell fraction and lower clonality in PBMC when measured early on-treatment, and higher TIL fraction in primary tumor constituted promising biomarkers of response. Pts with higher TIL fractions were more likely to have tumor-associated clones detectable in blood, suggesting these may be useful for tracking the immune response. These findings warrant validation in an independent cohort and exploration with other immunotherapeutics. Clinical trial information: NCT01608594