Two siblings who developed adult T-cell leukemia (ATL) with unusual clinical courses are presented. The brother showed spontaneous remissions at the beginning stage of the disease of 6 years' duration, and the sister remains free of complaints for 6 years without chemotherapy. The patients and 2 of 12 healthy members of their family examined had serum antibodies against ATL-associated antigens (ATLA). The expression of ATLA was observed in the cultured lymphocytes from the patients and one of the family members.