HTLV-1 proviral load is a risk marker for HAM/TSP, but it is insufficient to determine the disease outcome. HTLV-1 Tax and HBZ proteins have been implicated in HAM/TSP pathogenesis in inducing cell proliferation and cytotoxic T lymphocytes response.To quantify the expression of tax and HBZ mRNA in asymptomatic carriers (AC) and HAM patients, and to investigate their association with HAM/TSP.We quantified the expression of HTLV-1 tax and HBZ mRNA in 37 AC and 26 HAM patients classified according to proviral load as low (AC L and HAM L : <1% infected cells) or high (AC H and HAM H : >1%).The AC L subgroup showed the lowest frequency of individuals expressing tax mRNA in comparison with AC H , HAM L and HAM H , and tax mRNA load normalized by proviral load was significantly lower in the AC L . In turn, normalized HBZ mRNA expression was similar in all subgroups. Both tax and HBZ mRNA expression were moderately correlated with proviral load in AC (r=0.6, p<0.001) and were weaker in HAM (r=0.4, p<0.05). In contrast, the correlation between tax and HBZ mRNA load was moderate in AC (r=0.5, p=0.001) and was much stronger in HAM (r=0.8, p<0.001). In addition, HBZ mRNA load, but not tax, was significantly associated with motor disability in HAM patients (p=0.036).The expression of tax mRNA seems to be best to estimate the risk of HAM/TSP, whereas HBZ mRNA appears to be a surrogate marker to disease progression, indicating that they have important but distinct roles in HAM/TSP pathogenesis.