Serum samples from 76 patients with neutropenia and 34 control subjects were analyzed for levels of granulocyte colony-stimulating factor, granulocyte-macrophage colony-stimulating factor, interleukin 1 alpha, and tumor necrosis factor beta. Clinical correlates and duration of neutropenia were determined insofar as possible. Systemic acute inflammatory disease was present in only two patients. In most cases, the neutropenia was considered idiopathic or medication related. Significantly elevated serum granulocyte colony-stimulating factor levels were found in the patient group, regardless of the apparent cause of the neutropenia. Increased levels of granulocyte-macrophage colony-stimulating factor, interleukin 1, and tumor necrosis factor were seen in only one patient with sepsis.