A case of apparent association of dextran 70 therapy with reduced factor VIII activity in a 24-year-old white woman with thrombotic thrombocytopenic purpura (TTP) is reported. Diagnosis of TTP was based on clinical and laboratory data consistent with transient neurological deficits, thrombocytopenia and hemolytic anemia. Intravenous dextran 70 (250 ml every 12 hours) and oral prednisone (80 mg daily) were given for TTP. Additional oral medications included codeine, Maalox and Tabron. When assay showed a factor VIII activity of 14% on the third day of dextran 70 administration, the drug was discontinued and oral administration of dipyridamole (100 mg four times daily) was begun. Factor VIII activity increased to 82% on day six. Dextran 70 (125 ml every 12 hours) was resumed on day seven following a splenectomy on the previous day. When factor VIII activity subsequently decreased from 75% to 29% within two days, dextran 70 was again discontinued and dipyridamole reinstituted at its earlier dosage. The patient's discharge medications included dipyridamole and 60 mg of prednisone daily (to be tapered). Reports of dextran 70-induced hemostatic defects involving platelets and factor VIII are reviewed. Dextran will form, with factors I and VIII, cryoprecipitates that may reduce the activity of factor VIII. It is suggested that patients receiving dextrans be monitored with partial thromboplastin times and, if available, with assays of factor VIII activity.