Hypofibrinogenemia (plasma fibrinogen level <150 mg/dl) is occasionally observed after allogeneic hematopoietic stem cell transplantation, and its etiology is often difficult to determine. We herein report that steroids administered for the treatment of graft-versus-host disease (GVHD) are associated with the development of hypofibrinogenemia. We retrospectively analyzed the plasma fibrinogen (Fg) levels in 15 consecutive patients who had been administered 1 mg/kg/day (1 mg/kg group) or 2 mg/kg/day (2 mg/kg group) methylprednisolone for the treatment of Grade II to IV acute GVHD. Hypofibrinogenemia had developed in 8 of the 15 patients (53%) by day 50 after the start of steroid treatment, and was observed in 2 of 6 patients in the 1 mg/kg group and 6 of 9 in the 2 mg/kg group. A significant decrease in the Fg level was observed in the 2 mg/kg group (the median value before starting steroid treatment and that on the 20th day after starting steroid treatment were 506 mg/dl and 180 mg/dl, respectively, P=0.0013). Other possible causes of hypofibrinogenemia, including liver dysfunction or disseminated intravascular coagulation, were confirmed in only 3 patients during the observation period. In conclusion, hypofibrinogenemia commonly occurs in patients treated with steroids, especially those administered 2 mg/kg/day methylprednisolone for the treatment of GVHD.