Seroepidemiological studies have shown previously that cancer patients are less likely to have antibodies against the tumour suppressive adeno-associated virus (AAV) than control groups. To examine the influence of AAV infection on the development of adult T-cell leukaemia lymphoma (ATLL), an endemic disease in Southern Japan that is caused by infection with the human T-cell leukaemia virus type 1 (HTLV-I), the prevalence of serum antibodies to AAV type 2 (AAV-2) was tested in healthy HTLV-I carriers (n = 39) and patients with ATLL (n = 31). The results showed a significant difference in AAV-2 seropositivity between the two groups: Only 29% of the ATLL patients had IgG antibodies against AAV-2, whereas 84.6% of the healthy HTLV-I carriers were seropositive. Analysis of total serum IgG and antibodies against the Epstein-Barr virus (EBV) EBNA1 antigen showed that the lack of AAV antibodies in patients was not due to an ATLL-associated immune deficiency. The lower level of AAV-2 seropositivity in ATLL-patients may indicate that AAV-2 antibody-positive HTLV-I carriers might be less likely to develop ATLL or that loss of AAV-2 antibodies may parallel the development of disease. Copyright 2001 Wiley-Liss, Inc.