PURPOSE: To review the biologic origin, functional characteristics, and current and potential clinical applications of a novel marker of immune system activation, the soluble interleukin-2 receptor (sIL-2R). DATA IDENTIFICATION: Studies reported since 1985 were identified by a computer search using MEDLINE as well as from bibliographies of published work. STUDY SELECTION: Sixty-two reports on the clinical applications of the sIL-2R, largely from peer-reviewed journals, were identified. These reports address the utility and significance of sIL-2R measurements in various conditions. Basic scientific investigations delineating the biochemical and molecular features of the human interleukin-2 receptor complex and the sIL-2R protein were reviewed. DATA EXTRACTION: The validity of sIL-2R quantitation as an index of in-vivo immune system activation and its usefulness as a measure of disease activity and outcome were examined. RESULTS OF DATA ANALYSIS: The quantitation of sIL-2R, a novel laboratory measure of in-vivo immune system activation, correlates reliably with disease activity in autoimmune inflammatory disorders, transplantation rejection, and specific infectious disorders. Markedly elevated serum sIL-2R levels are a particularly prominent feature of certain hematologic malignancies, such as human T lymphotropic retrovirus type I-associated adult T-cell leukemia and hairy cell leukemia, reflecting tumor burden and response to therapy. The sIL-2R level at disease onset may also reliably predict long-term prognosis in non-Hodgkin lymphoma, and it appears to provide an additional serologic measure in the assessment of clinical progression in patients with human immunodeficiency virus infection and the acquired immunodeficiency syndrome. CONCLUSIONS: Studies have suggested that sIL-2R levels offer a rapid, reliable, and noninvasive measure of disease activity, response to therapy, and, in some cases, prognosis in a broad spectrum of conditions associated with T- or B-cell immune activation.