Sixteen families, each with two or more cases of adult T-cell leukemia or lymphoma were found in the Nagasaki district. Eight of the families had a parent with lymphoma. In the other eight families siblings were involved. Four families with sibling cases are presented in detail. Antibody titres to adult T-cell leukemia associated antigen (ATLA) in cases of ATL, CTL, T-CLL, and pre-ATL cases in Nagasaki were all positive. Of the non-leukemic T-cell malignant lymphoma 62.5 per cent were positive for antibody. The positive rate in healthy spouses and siblings of ATLL patients for ATLA antibody was high (67.5 per cent and 40 per cent respectively). The possibility of ATLV infection through spouses or from mother to child and the meaning of the high familial incidence of ATLL is discussed.