Extracorporeal photopheresis (ECP) is beneficial in patients with T-cell-mediated disorders, including GvHD, but the underlying immunological mechanisms are incompletely understood. Myeloid-derived suppressor cells (MDSCs) are innate immune cells characterized by their capacity to suppress T-cell proliferation. We quantified MDSCs by flow cytometry in peripheral blood from patients after BMT with GvHD before and after ECP treatment, patients after BMT but without GvHD and age-matched healthy controls. MDSC functionality was analyzed using T-cell proliferation, cytokine release and arginase activity. GvHD patients showed increased baseline percentages of neutrophilic MDSCs (PMN-MDSCs) compared with healthy controls and patients after BMT without GvHD. ECP treatment in GvHD patients rapidly increased circulating percentages of PMN-MDSCs. Functionally, PMN-MDSCs efficiently dampened Th1 and Th17 responses and were paralleled by an increase of cellular and extracellular arginase activity. Following ECP longitudinally over 16 weeks, two GvHD responder subgroups were identified, with group one continuously increasing PMN-MDSCs and group two with stable or decreasing PMN-MDSCs over time. This study demonstrates for the first time that ECP increases T-cell-dampening PMN-MDSCs in GvHD patients, a finding that should be confirmed in larger series of GvHD patients.