Two patients diagnosed as having acute promyelocytic leukemia (APL) and disseminated intravascular coagulation (DIC) were closely followed from the day of admission until completion of the first course of chemotherapy with serial coagulation studies including plasma levels of functional antithrombin III activity (AT III) by fluorometric analysis and anti-activated Factor X activity (anti-Xa) by coagulation assay. Both patients were treated with intravenous heparin and the presence of heparin in plasma was followed by the thrombin time. Consistently normal levels of AT III (greater than 80%) were found despite evidence of intravascular coagulation. However, plasma levels of anti-Xa were often low (less than 70%) and increased only in the presence of heparin. The significance of these results in relationship to heparin therapy for disseminated intravascular coagulation of APL is discussed.