GVHD is a major complication following allogeneic hematopoietic SCT, and is associated with substantial morbidity and mortality. Based on the results of our previous clinical study with females treated with human chorionic gonadotropin (hCG) as preconditioning therapy for in vitro fertilization, we hypothesized that low-dose hCG stimulates indoleamine-2,3-dioxygenase (IDO), IL 10 and regulatory T cells (Treg), thereby suppressing clinical manifestations of chronic GVHD. Active chronic GVHD localized at skin, subcutaneous tissue, joints or gastrointestinal tract that was refractory or intolerant to glucocorticoid therapy improved substantially in 12 of 20 patients treated with hCG for 8 weeks (off-label), enabling a glucocorticoid dose reduction of 28% (average). Twelve of 19 patients with chronic GVHD of the skin responded to hCG therapy with a reduction of 25% (average) in their total skin score. HCG treatment increased IDO expression at median by sevenfold in peripheral mononuclear cells and IL10 levels in serum up to twofold at median from the pretreatment baseline. Further, an expansion of the Treg cell population was measured in one patient, which is also associated with the induction of tolerance. This novel application of low-dose hCG was well tolerated and is of clinical interest for GVHD treatment.