The prospective feasibility study was designed to determine whether treatment with antithrombin (AT) concentrates could be used for patients with severe preeclampsia to provide clinical efficacy without the full systemic antihypertensive drug. Twenty-nine severe preeclamptic patients (24 to 36 weeks of gestation, gestosis index [GI] greater than or equal to 6 points) were randomized to receive AT (1500 U/day) or not. AT was given intravenously once daily for 7 consecutive days. Twenty-four hours of continuous infusion of 5000 U/day of unfractionated heparin was given simultaneously in both groups. There were no statistical differences in the clinical profiles of the two groups. Maternal symptoms were evaluated from the difference of GI before and after treatment, and fetal findings were evaluated from the changes of the biophysical profile score. Improvement was significantly greater in the AT group for both the GI (P = 0.046) and the biophysical profile score (P = 0.022). The improvement of coagulation parameters was also evaluated. The improvement of plasmin-plasmin inhibitor complex (PPIC), D-dimer levels, and platelet counts was observed in the AT group compared with the non-AT group. The coagulation index as a composite coagulation test parameter showed a significantly improvement in the AT group. No adverse events related to AT were observed. Given that AT plus heparin was significantly better than treatment with heparin alone for improvement of both GI and biophysical profile of infant, it is suggested that therapy with AT alone might be effective enough for severe preeclampsia.