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 0%-2%. The serum lactate dehydrogenase (LDH) value was within normal range and was not associated with hypercalcemia; lymphadenopathy, hepatosplenomegaly, and bone marrow infiltration were 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 anti-ATLA antibodies were positive; none 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 of cryptococcus meningitis. Based on clinical and pathologic differences, we believe that these cases should be distinguished from typical ATL cases for the purposes of prognosis and treatment.