A case of adult T-cell leukemia (ATL) in a 23-yr-old Japanese man without immunoglobulin (Ig)G antibody to ATL-associated antigen (ATLA) (anti-ATLA) is presented. Comparative studies revealed that there were no differences between the present case and the anti-ATLA-positive ATL cases, except for the onset of the former at a younger age. Serologic and epidemiologic studies were performed to determine whether in this case the disease was still dependent on ATLA. As serum IgE and IgA had been persistently elevated, the serum was examined for these antibodies to ATLA. Furthermore, after the patient's serum was treated with glycine buffer to dissociate possibly existing immune complexes, it was assayed for antibodies (IgG, IgE and IgA) to ATLA. However, no antibody activity could be demonstrated. Sero-epidemiologic studies revealed that there were no anti-ATLA-positive healthy adults (ATLA-HA) in either his family or among the residents of his birthplace, whereas some ATLA-HA have been found among the residents of the birthplaces of other ATL patients. Although further investigation is necessary, it seems reasonable for the present to consider that a minor group of ATL patients independent of ATL virus may exist.