Adult T-cell leukemia/lymphoma (ATLL) is caused by latent human T-lymphotropic virus-1 (HTLV-1) infection. To clarify the molecular mechanism underlying leukemogenesis after viral infection, we precisely mapped 605 chromosomal breakpoints in 61 ATLL cases by spectral karyotyping and identified frequent chromosomal breakpoints in 10p11, 14q11, and 14q32. Single nucleotide polymorphism (SNP) array–comparative genomic hybridization (CGH), genetic, and expression analyses of the genes mapped within a common breakpoint cluster region in 10p11.2 revealed that in ATLL cells, transcription factor 8 (TCF8) was frequently disrupted by several mechanisms, including mainly epigenetic dysregulation. TCF8 mutant mice frequently developed invasive CD4⁺ T-cell lymphomas in the thymus or in ascitic fluid in vivo. Down-regulation of TCF8 expression in ATLL cells in vitro was associated with resistance to transforming growth factor β1 (TGF-β1), a well-known characteristic of ATLL cells, suggesting that escape from TGF-β1–mediated growth inhibition is important in the pathogenesis of ATLL. These findings indicate that TCF8 has a tumor suppressor role in ATLL.