We have observed five patients with smoldering adult T-cell leukemia (ATL) who had skin lesions as premonitory symptoms. The illness developed slowly but flared up after several years. Skin lesions appeared in the form of erythema, papules or nodules. Infiltration of the skin by ATL cells was slight, and the proportion of ATL cells in the peripheral blood was from 0% to 2%. The serum lactic dehydrogenase value was within normal range, and was not associated with hypercalcemia, lymphadenopathy, or hepatosplenomegaly, and bone marrow infiltration was very slight. In most cases, hypergammaglobulinemia was seen, and in one case monoclonal hypergammaglobulinemia was observed. All five patients had lived in an area in which ATL was endemic, and their sera were positive for anti-ATL-associated antigen antibodies. None of them had ever received a blood transfusion. One patient developed typical ATL after more than 13 yr of illness, and died of renal insufficiency. Another patient developed typical ATL after 5 yr of illness, and died or cryptococcus meningitis. These cases were clinically and pathologically different from typical ATL cases already reported, and we feel it necessary to make distinctions from the viewpoints of prognosis and treatment. In discussing these cases, we compared smoldering ATL with typical ATL, and deliberated upon the causes of both.