Hairy cell leukemia (HCL) is a well-recognized chronic lymphoproliferative disorder of B cell lineage, which may be regulated by growth factors including cytokines and cytokine antagonists. Previous studies have shown a good correlation between circulating soluble interleukin-2 receptor (sIL-2R) levels and disease activity and response to therapy was always associated with a decrease in sIL-2R levels. The interleukin-1 (IL-1) family of agonists and antagonists may also be involved in the regulation of hematopoietic malignancies. In the present study, we evaluated members of the IL-1 family (IL-1beta, IL-1 receptor antagonist (IL-1Ra) and IL-1 soluble receptors Type I and Type II (IL-1sRI and IL-1sRII) in 23 patients with HCL. Patients were classified according to the clinical state of their disease. Most were treated with 2-chloro-2'-deoxyadercosine (2-CDA) and treatment was associated with a significant decrease in the serum levels of sIL-2R, IL-1beta and IL-1sRII in patients achieving a complete or partial response. In contrast to the above, levels of IL-1Ra increased during response to treatment and clinical response to 2-CDA was associated with an increase of 122% in IL-1Ra levels, in parallel with a decrease of 63% in IL-1beta and 47% in IL-1sRII levels. These results suggest that the balance between IL-1beta, IL-2 and their soluble receptors or antagonists may be involved in the pathogenesis and immunoregulation of HCL. Serum levels of these cytokines may therefore be used to monitor therapeutic efficacy of therapy in HCL and to detect any residual disease.