[en] A determinant of human T-lymphotropic virus-1 (HTLV-1)–associated myelopathy/ tropical spastic paraparesis (HAM/TSP) development is the HTLV-1–infected cell burden. Viral proteins Tax and HBZ, encoded by the sense and antisense strands of th pX region, respectively, play key roles in HTLV-1 persistence. Tax drives CD4 - cel clonal expansion and is the immunodominant viral antigen recognized by the immune response. Valproate (2-n-propylpentanoic acid, VPA), a histone deacetylase inhibitor, was thought to trigger Tax expression, thereby exposing the latent HTLV-1 reservoir to immune destruction. We evaluated the impact of VPA on Tax, Gag, and HBZ expressions in cultured lymphocytes from HTLV-1 asymptomatic carriers and HAM/TSP patients. Approximately one-fifth of proviruspositive CD4 T cells spontaneously became Tax-positive, but this fraction rose to two-thirds of Tax positive–infected cells when cultured with VPA. Valproate enhanced Gag-p19 release. Tax- and GagmRNA levels peaked spontaneously, before declining concomitantly to HBZmRNA increase. VPA enhanced and prolonged Tax-mRNA expression, whereas it blocked HBZ expression. Our findings suggest that, in addition to modulating Tax expression, another mechanism involving HBZ repression might determine the outcome ofVPAtreatment on HTLV-1–infected– cell proliferation and survival.