Abstract Serum concentrations of soluble interleukin 2 receptors (sIL 2R) were measured by an enzyme-linked immunosorbent assay (ELISA) in 30 patients with adult T cell leukemia (ATL), in 9 patients with other hematopoietic malignancies, and in 17 asymptomatic individuals seropositive for human T cell leukemia virus type I (HTLV-I). Sixty HTLV-I seronegative, age-matched controls showed a normal range of form 63.2 to 480.8 U/mL. All asymptomatic carriers of HTLV-I had sIL 2R in their sera within the normal range. sIL 2R in sera was not related to the anti-HTLV-I antibody titer. Eleven patients with acute ATL, a clinical phenotype with median survival rate of 4.4 months, had markedly elevated sIL 2R (11,100 to 99,000 U/mL), but eight patients with smoldering ATL had low sIL 2R values (less than 480.8 U/mL) comparable to controls. Eleven patients with chronic ATL had intermediate elevated levels of sIL 2R (480.8 to 37,300.0 U/mL). Serum levels of sIL 2R correlated with the number of ATL cells (r = 0.812) and CD25-positive cells (r = 0.725) circulating in the peripheral blood. Longitudinal studies performed in four patients with ATL showed significant correlation between serum concentration of sIL 2R and activity of the malignancy. These findings suggest that the level of sIL 2R in serum indicated tumor load and, possibly, prognosis.