A rare case of thrombotic microangiopathy in a patient with hemophagocytic syndrome is reported. An 18-year-old girl was admitted following prolonged fever, watery diarrhea, abdominal discomfort, and a 2-week history of rhinorrhea, cough, and painful cervical lymph nodes. Anemia, thrombocytopenia, jaundice, hepatomegaly, and mild azotemia developed within 2 weeks of admission. The diagnosis of a reactive hemophagocytic syndrome, probably secondary to infection, was made based on the findings of bone marrow examination. Extensive investigation failed to identify a causative agent. The disease initially responded rapidly to intravenous steroids and high-dose immunoglobulin therapy but relapsed soon after tapering of the steroids. Although her condition improved again on resumption of treatment with high-dose steroids, nephrotic range proteinuria and microscopic hematuria developed after the steroids were tapered. Fragmented erythrocytes were seen in peripheral blood with elevated serum lactate dehydrogenase and decreased serum haptoglobin concentrations. The results of subsequent renal pathology examination were also compatible with thrombotic microangiopathy. The disease course finally stabilized after a course of pulse methylprednisolone therapy. Immune hyperactivity, particularly hypercytokinemia and monocyte hyperactivity, could have accounted for the development of thrombotic microangiopathy in this case. Only strong immunosuppressive therapy can control such disease activity.