Purpose: FOLFIRINOX has demonstrated promising results for patients with pancreatic ductal adenocarcinoma (PDAC). Chemotherapy- induced immunogenic cell death can prime antitumor immune responses. We therefore performed high-dimensional profiling of immune cell subsets in peripheral blood to evaluate the impact of FOLFIRINOX on the immune system. Experimental Design: Peripheral blood mononuclear cells (PBMC) were obtained from treatment-naïve (n=20) and FOLFIRINOX- treated patients (n =19) with primary PDAC tumors at the time of resection. PBMCs were characterized by 36 markers using mass cytometry by time of flight (CyTOF). Results: Compared with treatment-naïve patients, FOLFIRINOX- treated patients showed distinct immune profiles, including significantly decreased inflammatory monocytes and regulatory T cells (Treg), increased Th1 cells, and decreased Th2 cells. Notably, both monocytes and Treg expressed high levels of immune suppression- associated CD39, and the total CD39+ cell population was significantly lower in FOLFIRINOX-treated patients compared with untreated patients. Cellular alterations observed in responders to FOLFIRINOX included a significantly decreased frequency of Treg, an increased frequency of total CD8 T cells, and an increased frequency of CD27-Tbet+ effector/effector memory subsets of CD4 and CD8 T cells. Conclusions: Our study reveals that neoadjuvant chemotherapy with FOLFIRINOX enhances effector T cells and downregulates suppressor cells. These data indicate that FOLFIRINOX neoadjuvant therapy may improve immune therapy and clinical outcome in patients with PDAC.