In patients with hairy cell leukemia (HCL), we measured serum levels of monocyte colony-stimulating factor (M-CSF), interleukin-6 (IL-6), and erythropoietin during various degrees of pancytopenia characteristic for this disease. Serial sera from 12 HCL patients during various stages of the disease were analyzed. No correlation was found between the levels of M-CSF or IL-6 and the numbers of circulating monocytes or platelets, normal values of M-CSF (4 to 10 mg/l), and IL-6 (3-50 U/ml) being detected during all stages of the disease. In contrast, erythropoietin levels were inversely related with the hemoglobin concentration (r = -0.79), indicating the presence of a normal feedback mechanism for this factor in patients with HCL.