Serum levels of immunosuppressive acidic protein (IAP) were measured in patients with adult T-cell leukemia (ATL) in order to clarify its significance in this disease. The mean levels in patients with both acute (854 +/- 404 micrograms/ml) and chronic ATL (439 +/- 103 micrograms/ml) were significantly higher than in sera of healthy controls (367 +/- 104 micrograms/ml). However, mean levels in patients with smoldering ATL, healthy T-cell lymphotropic virus type 1 (HTLV-1) carriers and healthy controls showed no differences. Levels in crisis in chronic and smoldering ATL were similar to those in patients with acute ATL. Serial measurements of serum IAP in a number of patients revealed that the levels reflected each patient's clinical course, suggesting a potential for use in evaluating the effects of chemotherapy.