Adult T-cell leukemia/lymphoma (ATLL) is a T-cell neoplasm caused by human T-cell leukemia virus type I (HTLV-I). ATLL is classified into four clinical subtypes based on the clinical manifestation: acute, lymphoma, chronic and smoldering. Approximately half of chronic type ATLL cases progressed to the acute type. We previously demonstrated that genomic alterations related to the cell cycle de-regulation such as CDKN2A and immune escape such as CD58 alteration can serve as predictive biomarkers for acute transformation of the chronic type. Although alteration of TP53, which is known to be a major regulator of cell cycle, has been identified in several types of cancers including acute type ATLL, no copy number alteration of TP53 was found in the chronic type by array comparative genomic hybridization. In the present study, mutation of TP53 was further analyzed by sequencing for these cases as well as HTLV-I carriers with oligo-clonality. However, no TP53 mutation was identified. These results suggested that TP53 mutation plays a role for the later stage of ATLL development.