Using an enzyme-linked immunosorbent assay, we measured the level of soluble interleukin-2 receptors (sIL-2Rs) in the serum of 50 normal controls, 48 human carriers of T cell leukemia virus type I (HTLV-I) antibody who had no symptoms, 11 patients with HTLV-I-associated myelopathy (HAM), and 39 patients with adult T cell leukemia (ATL) (four smoldering, 10 chronic, nine lymphoma, and 16 acute type). The highest levels of sIL-2R were observed in patients with acute and lymphoma-type ATL, as opposed to those with chronic and smoldering-type ATL. The levels of sIL-2R in patients with HAM were roughly between those of smoldering ATL and healthy HTLV-I carriers. Serum sIL-2R levels in healthy carriers were also elevated compared with those in normal controls. These observations suggest that the measurement of sIL-2R levels in patients with ATL can be useful as a noninvasive measure of tumor burden and should aid in the understanding of the natural history of HTLV-I infection leading to the development of ATL.