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Although there are National Institutes of Health consensus criteria for the global assessment of chronic graft-versus-host disease, no validated biomarkers have been established for this disease. Furthermore, whereas the role of T cells, B cells, and dendritic cells in chronic graft-versus-host disease has been established, the contribution of monocytes has not been clearly addressed. Using an enzyme-linked immunospot assay, we measured the spot-forming cells for interferon-γ, interleukin-4, interleukin-10, and interleukin-17 in unstimulated peripheral blood of patients following allogeneic hematopoietic stem cell transplantation. Other immunological examinations, including skin biopsy, were also done. Fifty-seven patients were enrolled. Interleukin-10 spot-forming cells were evaluable for therapeutic monitoring in 16 patients with chronic graft-versus-host disease. The number of interleukin-10 spot-forming cells in patients with active chronic graft-versus-host disease was significantly higher than the number in those with no or inactive chronic graft-versus-host disease. Interleukin-10 was predominantly produced by monocytes. CD29 expression on monocytes in patients with active chronic graft-versus-host disease was elevated. The level of plasma fibronectin, a ligand of CD29, correlated with the number of interleukin-10 spot-forming cells. Immunohistochemical analysis of the skin in active chronic graft-versus-host disease showed that infiltrating CD29(+) monocytes might produce interleukin-10. A novel biomarker, interleukin-10 spot-forming cells, shows promise as both a diagnostic and prognostic indicator for chronic graft-versus-host disease, and may allow for early intervention prior to the onset of the disease. Measurement of interleukin-10 spot-forming cells would be helpful in clinical trials and in patients' management.