Three patients with pre-adult T-cell leukemia (ATL) are reported. All of them were seropositive to ATL-associated antigens (ATLA) and had 3-30% abnormal lymphocytes with cleaved or lobulated nuclei in the peripheral blood in the absence of clinical symptoms of ATL. All the data on immunological tests were normal except for negative purified protein derivative skin tests. In Case 1 the patient was an elder sister of an ATL patient. Four other members of this family were also anti-ATLA positive without abnormal cells in the blood. In Cases 1 and 2, ATLA and ATL virus (ATLV) were expressed in the lymphocytes cultured with phytohemagglutinin or T-cell growth factor. Moreover, an ATLV-producing cell line was established from Case 1 by mixed lymphocyte culture with lymphocytes obtained from a normal anti-ATLA negative person. Investigation of pre-ATL cases may clarify factors leading to the development of ATL in ATLV-infected persons.