A patient with acute adult T-cell leukemia (ATL) in whom spontaneous remission was observed without any specific treatment having been given is described. The abnormal cell phenotype was CD4+, CD45RO+ and CD8-. As the number of abnormal cells decreased, CD4+ cell count decreased and CD8+ cells and CD45RA+ cells increased to normal levels (45 and 77%, respectively). Further, the number of cells with CD45RO antigen of intermediate fluorescence intensity increased. Five months after admission, we assessed the patient as being in a state of complete clinical remission; no abnormal cells were detected in peripheral blood, lymph node enlargement had disappeared and the serum chemistry was normal. When the abnormal cells in peripheral blood had disappeared, Southern blot analysis for HTLV-I proviral DNA still revealed a weak monoclonal band with Eco Rl digestion, and HTLV-I proviral DNA was detected by polymerase chain reaction analysis. Thus, it appeared that very few abnormal cells persisted although the laboratory findings for ATL were normal. Our case could contribute to the understanding of the mechanism that underlies spontaneous remission in ATL.