Human T-cell leukemia/lymphoma virus type 1 (HTLV-1) infects 15-20 millions individuals worldwide. This oncoretrovirus can be transmitted through 3 ways: horizontally, vertically (mother to child) and via blood transfusion. HTLV-1 causes 2 major diseases: adult T-cell leukemia/lymphoma (ATLL) and tropical spastic paraparesis/HTLV-1-associated myelopathy. Tax is a 40-kDa phosphoprotein that is encoded by the pX region of the virus. Several lines of evidence have demonstrated a central role for this protein in the immortalization or transformation of the HTLV-1 infected cells. Apart from its ability to drive transcription from the viral promoter, it also deregulates the cell cycle, inhibits apoptosis, has an effect on the maintenance of the genomic stability and induces the production of several cytokines. In addition, several arguments strongly suggest the existence of host genetic factors, that could be involved in the HTLV-1 infection as well as in the development of ATLL among HTLV-1 infected individuals. ATLL can be classified into 4 major subtypes: a smoldering type, a chronic type, a lymphoma type and a leukemic type. The demonstration by Southern blot analysis of the clonal integration of an HTLV-1 provirus in the tumoral cells represents the gold-standard to define biologically ATLL. The survival rate of ATLL patients, especially those who develop the acute leukemic or lymphomas forms, is very poor, and such clonal malignant CD4 expansion remains one of the most severe lymphoproliferations.