Bone marrow (BM) and peripheral blood (PB) cells from patients with juvenile chronic myelogenous leukemia (JCML) exhibit spontaneous in vitro proliferation. Several cytokines including granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-1 beta (IL-1 beta), and tumor necrosis factor alpha (TNF alpha) have been implicated in supporting the growth of leukemic monocyte-macrophage colonies either by autocrine or paracrine pathways. In seven untreated JCML patients, we investigated the role of IL-1 in the spontaneous growth of these cells by specifically blocking IL-1 receptors. The IL-1 receptor antagonist (IL-1 Ra) was added to the clonogenic assays, and in each case significant (mean = 63%, range = 35% to 82%) inhibition of spontaneous proliferation was observed. Uncultured circulating cells from PB or BM of four out of five patients expressed IL-1 beta-specific mRNA and secreted the protein into the culture supernatants. Moreover, by means of reverse transcriptase-polymerase chain reaction (RT-PCR), we demonstrated that most of the spontaneously growing leukemic colony- forming unit cells (CFU-C) obtained from BM cells of two patients were positive for the presence of the IL-1 beta-specific mRNA. Despite the presence of a measurable amount of GM-CSF in JCML cell culture supernatants, GM-CSF-specific mRNA in CFU-C cells of four cases was not detected by RT-PCR. These data further support a central role for IL-1 beta in the pathogenesis of JCML and suggest that the use of IL-1 Ra could represent a novel therapeutic strategy against this disorder.