The clinical and pathological findings in four cases of fulminant hepatic failure due to massive infiltration of the liver by acute leukemia or lymphoma are reported. Liver abnormalities were found simultaneously with or led to the discovery of hematologic malignancies, and consisted of marked hepatomegaly and severe hepatocellular insufficiency associated with hyperlactatemia. The blood malignancies were peculiar in their fast cellular growth and large tumor mass. Evolution was rapidly fatal in all these cases. In another patient, marked hepatomegaly and hyperlactatemia revealed the presence of a widespread lymphoma before the appearance of hepatocellular insufficiency. Immediate chemotherapy was instituted, and complete remission without hepatic complication was obtained. It is suggested that malignant hematological diseases with fast cellular growth may present as fulminant hepatic failure. In order to avoid a rapidly fatal outcome secondary to liver failure and metabolic disorders, early recognition of these malignancies is necessary so as to assure prompt administration of appropriate chemotherapy.