Résumé : Only a small proportion of patients infected by HTLV-1 will develop Adult T-cell leukemia (ATL). ATL xis not specific clinically and is characterized by a prolonged latency which leads us to suppose that several factors are involved. The particular mode of replication of HTLV-1 through clonal expansion of the infected cells provokes, at the asymptomatic stage, a chronic lymphoid proliferation at minimum. The regulating proteins coded by the pX region of the provirus play an essential part in the regulation of viral replication. Replication and transformation seem to be intricate in the natural history of the infection. By maintaining the chronic proliferation of the infected cells, the proteins of the pX region are involved in the process of triggering the transformation. Others factors seem to be involved in the growth of tumors, i.e. deletions of the 5' part of the provirus, infectious cofactors... At the last stage of ATL, some non-specific genetic events like p53 inactivation are found. The combined use of zidovudine and interferon would seem to be a new efficient treatment of ATL. The synergy of anti-tumoral action of both these molecules seems to be more involved in the efficiency of that treatment than the inhibiting effect of zidovudine on the reverse transcriptase of HTLV-1.