Protein C, a newly identified inhibitor of blood coagulation, was measured immunologically in 58 patients with untreated acute leukemias and compared with that of normal subjects. On the average, slightly lower values were found. However, the 17 patients with overt laboratory pictures of decompensated disseminated intravascular coagulation (DIC), including 11 cases with acute promyelocytic leukemia, had protein C concentrations no lower than those of the remaining 41 patients without DIC. Antithrombin III activity and antigen were normal and, like protein C, not lowered in DIC. The concentrations of both proteins were closely correlated with changes in the indexes for liver synthetic function. A subgroup of 13 patients with hyperleukocytic leukemias had lower protein C and antithrombin III, in line with the more compromised synthetic function of their livers. Our findings indicate that liver impairment rather than DIC is the main cause of the changes in the two naturally occurring inhibitors of blood coagulation.