Reversible acetylation of the histone tails plays an important role in the control of specific gene expression. Mounting evidence has established that histone deacetylase inhibitors such as Valproic Acid (VPA)selectively induce cellular differentiation and apoptosis in variety of cancer cells. In a single-center, one year open-label trial, 19 HAM/TSP patients were treated with oral doses of VPA (20mg/Kg/day). Primary end-points were the therapeutic safety and the effect on HTLV-1 proviral load (a significant and sustained decrease was expected). Secondary end-point was the neurological status before and after one-year treatment.