Myeloid-derived suppressor cells (MDSCs) have emerged as a heterogeneic immunoregulatory population that can expand in response to inflammatory signals. Predominantly studied in cancer, MDSCs suppress T cells utilizing various mechanisms. In allogeneic hematopoietic stem cell transplantation (allo-HSCT) therapy-related toxicity and alloreactivity increase inflammatory cytokines that might favor an MDSC accumulation. To address this question, circulating CD14(+)HLA-DR(low/neg) cells were studied retrospectively in 51 allo-HSCT patients. These cells represent one of the few well-described human MDSC subsets under physiological and pathological conditions. The frequency of CD14(+)HLA-DR(low/neg) cells was significantly increased after allo-HSCT, especially in patients with acute graft-versus-host disease. Compared to healthy donor cells they were pSTAT1(low) (phosphorylated signal transducer and activator of transcription) and indoleamine 2,3-dioxygenase (IDO)(high). Serum levels of granulocyte colony-stimulating factor and interleukin-6, which both have been linked to MDSC induction, correlated positively with the frequency of CD14(+)HLA-DR(low/neg) cells. In vitro dysfunction of patient T cells, such as reduced proliferative capacity or CD3ζ-chain expression, was rescued by blocking the IDO activity of CD14(+)HLA-DR(low/neg) cells. Overall, we identified a T-cell-suppressive monocytic population that expands after allo-HSCT. The mechanisms responsible for such accumulation remain to be elucidated. It will be of great interest to prospectively investigate the influence of these cells on the graft-versus-tumor and -host reaction.