Recently, several malignant cell types have been reported to express colony-stimulating factor-1 (CSF-1) transcripts; however, the clinical significance of CSF-1 in malignancy has not been investigated. Using a CSF-1 radioimmunoassay, we surveyed concentrations of biologically active CSF-1 in the peripheral blood of 316 patients with malignant and premalignant hematologic disorders; 75 had a myelodysplastic syndrome (MDS), 12 acute myelogenous leukemia (AML), 7 chronic myelogenous leukemia, 21 chronic lymphocytic leukemia (CLL), 106 non-Hodgkin's lymphoma (NHL; of low-, intermediate- and high-grade malignancy), 46 Hodgkin's disease (HD), 46 multiple myeloma (MM), and 3 monoclonal gammopathy of undetermined significance. Controls were 64 healthy subjects. The CSF-1 concentration was correlated with the type of disease, status of the disease, treatment status, and hematologic parameters. CSF-1 concentration was significantly elevated in 83.5% of the patients with active disease, and for each active disease group it was significantly greater (P less than .0001) than in the control. Thus, the high circulating CSF-1 concentration was not associated with a particular malignant phenotype or MDS subtype, but did correlate with the disease activity of both NHL and HD, and the tumor burden in MM, AML, and CLL. There was no correlation of the CSF-1 level with total counts of monocytes or neutrophils in patients with MDS or other malignancies. The cellular basis for the elevated circulating CSF-1 was not investigated. However, the results are consistent with the possibility that the premalignant or malignant cells themselves produce CSF-1 or regulate its production by normal cells.