Hemophagocytic syndrome (HPS) is a syndrome presenting with signs of persistent remittent fever, hepatosplenomegaly, pancytopenia, hepatic dysfunction, and disseminated intravascular coagulation (DIC) due to hypercytokinemia caused by activated T lymphocytes and macrophages. The mortality in adults is high and a small number of complicated cases during pregnancy have been reported. We report one HPS case that developed a remittent fever, leukocytopenia, and thrombocytopenia in the 21st week of pregnancy, and abnormal blood coagulation, hepatic dysfunction, and hypercytokinemia were found. Antibiotics and immunoglobulin were given but failed to improve clinical and laboratory findings. At the 24th week, the patient was diagnosed with DIG, and antithrombin (AT) concentrate was given. With the increase in plasma levels of AT, improvements were seen in both clinical signs and laboratory findings. Bone marrow biopsies were carried out, and a diagnosis of HPS was made. Preeclampsia developed in the 27th week and it became severe. Cesarean section was performed in the 29th week because of severe preeclampsia, intrauterine growth retardation (IUGR), and fetal distress. The courses of mother and newborn were uneventful. We discuss the mechanism of AT in the treatment of this syndrome and the association between this syndrome and severe preeclampsia. In conclusion, AT concentrate was very effective in suppressing cytokine prediction, and the possibility that severe preeclampsia developed because of hypercytokinemia, which may be one of the pathogeneses of severe preeclampsia and IUGR, was suggested.