Human T-cell leukemia virus type I (HTLV-I) is the etiological agent of adult T-cell leukemia (ATL), although the precise mechanisms involved in the transformation process have not yet been defined. Deregulated inhibition of apoptosis may facilitate the insurgence of leukemia. Survivin is an inhibitor of apoptosis and considered to play a role in oncogenesis. We investigated the expression of survivin in HTLV-I-infected T-cell lines and primary ATL cells. Northern blot analysis showed expression of survivin transcript in all 9 HTLV-I-positive T-cell lines. High survivin expression levels were detected in primary cells of patients with acute-type ATL, but no such expression was noted in the cells of chronic-type ATL or normal peripheral blood mononuclear cells. Treatment of an HTLV-I-infected T-cell line, MT-1, with survivin specific antisense oligonucleotide resulted in reduced cell growth. Our results suggest that expression of survivin may play an important role in the development and pathogenesis of ATL.