Background: Regional chemotherapy is known to yield high drug concentrations in the treated tumor area while keeping systemic drug concentrations low. Regional Chemotherapy can be performed as an intra arterial infusion or isolated perfusion. Known perfusion techniques are isolated limb perfusion, hypoxic pelvic perfusion, hypoxic abdominal perfusion, upper abdominal perfusion, and isolated thoracic perfusion, Only little is known on the effect of chemotherapy on immune cells, especially not if chemotherapy is applied regionally. Since more and more immuno oncologic treatment possibilities arise, that often are combined with chemotherapy, knowledge on the effects on immune cells is crucial. Since systemic chemotherapy often decreases the number of immune cells in general, the more targeted approach of regional chemotherapy is investigated in this study. Methods: In this work, we investigate subpopulations of T-lymphocytes, Natural Killer cells and B-lymphocytes during regional chemotherapy and report on their changes after cycles of intra-arterial chemotherapy infusion followed by chemofiltration. Surface phenotyping of cells from EDTA blood was performed with combinations of monoclonal antibodies (anti- CD3,CD16, CD56, CD45, CD4, CD19, CD8, CD28, HLA-DR, CD39, CD25, CD127, CD45RA, CD45RO) for FACS analysis. 58 Patients with advanced breast cancer, ovarian cancer, head and neck cancer, lung cancer, or bladder cancer received regional chemotherapy as an intra arterial infusion therapy or an isolated perfusion therapy. Results: Contrary to the mild immune cell depletion during the first two cycles a more activated immune status is seen after treatment cycle three. Non-significant increase or stable numbers are observed for T-Lymphocytes and its subsets of CD4+ T-helper cells, CD8+ cytotoxic T-cells, and terminal memory T-cells (CD8+/CD28-/CD57+). Significant increase of cell numbers is observed for B-lymphocytes, regulative CD8+ T-cells (CD8+/CD28-/CD57-), activated HLA-DR+ T cells, and Natural Killer cells while regulative CD4+ T cells are significantly decreased. Conclusions: The data show that regional chemotherapy has controversial effects on the immune cell subpopulations. As every chemotherapy, it does affect immune cells, however to a limited extend, but importantly has an activating potential. Immune activation prevalently takes place after the third cycle of therapy which might be a hint for auto-vaccination caused by phases of repeated tumor necrosis after locally high drug concentrations. Finding the optimal dosage of chemotherapeutics may be crucial to balance direct tumor response, immune toxicity and immune activation.