Four patients with advanced diffuse histiocytic lymphoma who developed lactic acidosis are described. All four patients demonstrated disturbed liver function tests. In two of the patients, the lactic acidosis was unresponsive to treatment. The third patient responded successfully to the early initiation of combination chemotherapy with achievement of a clinical remission and correction of the lactic acidosis. The fourth patient responded to the initiation of chemotherapy with abatement of his lactic acidosis, but expired probably as the result of a pulmonary embolus. It seems likely that extensive hepatic infiltration may be one of the factors contributing to lactic acidosis in patients with diffuse histiocytic lymphoma. The early initiation of antineoplastic therapy may be important in the management of patients with histiocytic lymphoma and lactic acidosis.