p16INK4A is a cyclin-dependent kinase inhibitor (CDKI), and regulates the cell cycle negatively. Recently, p16INK4A protein was shown to be encoded by the CDKN2 gene, which is identical to multiple tumor suppressor gene 1 (MTS1) on chromosome 9p21, where genetic alterations occur frequently in many malignant tumors. As the loss of p16INK4A function by genetic alterations leads to inappropriate progression of the cell cycle, the CDKN2 gene has been investigated intensively as a new candidate tumor suppressor gene in many malignant tumors. Adult T-cell leukemia/lymphoma (ATLL) is a peripheral T-cell malignancy associated with human T-cell lymphotrophic virus type 1 (HTLV-1). As the development to ATL is believed to require not only HTLV-1 infection but also accumulation of genetic alterations, we investigated the relationship between alterations in the CDKN2 gene and ATL. Alterations in the CDKN2 gene were detected in approximately 15 to 20% of ATL patients. Interestingly, most of the patients with CDKN2 gene alterations had the aggressive form of ATL. The CDKN2 gene appears to be the major tumor suppressor gene on chromosome 9p21, and alteration in this gene may play an important role during late stages in the transformation process induced by HTLV-1.