Immunosuppressive CD4+CD25(hi)FoxP3+ T cells (T(reg) cells) have been found at increased densities within the tumor microenvironment in many malignancies and interfere with protective antitumor immune responses. Osseous Ewing sarcomas (ESs) are thought to derive from a bone marrow (BM) mesenchymal cell of origin, and microscopic marrow involvement defines a subpopulation of patients at a high risk of relapse. We hypothesized that BM-resident T cells may contribute to a permissive milieu for immune escape of ESs. Using 6-color-flow cytometry, we investigated the pattern of immune cell subset distribution including NK cells, gammadelta T cells, central and effector memory CD8+ and CD4+ T cells as well as T cells with regulatory phenotype (T(reg) cells) in BM obtained at diagnosis from 45 primary or relapsed ES patients treated within standardized protocols. Although patients at relapse had an inverted CD4:CD8 T-cell ratio, neither CD8+ effector/memory T-cell subsets nor T(reg) cells significantly differed from patients at diagnosis. No significant associations of innate and effector/memory T-cell subpopulations with known risk factors were found, including age, gender, tumor site, primary metastases and histological tumor response. By contrast, T(reg) cells were found at significantly higher frequencies in patients with primary metastatic disease compared with localized ESs (5.0 vs. 3.3%, p = 0.01). Thus, increased BM T(reg) cells in patients with metastasized ES may reflect an immune escape mechanism that contributes to the development of metastatic disease. Immunotherapeutic strategies will have to adequately consider the regulatory milieu within areas of Ewing tumor-immune interactions.