Despite abundant activated Human T cell Lymphotropic Virus type 1 (HTLV-1)-specific Cytotoxic T Lymphocytes (CTL), patients with HTLV-1-Associated Myelopathy/Tropical Spastic Paraparesis (HAM/TSP) show a significantly higher frequency of infected cells than Asymptomatic Carriers (AC) of the virus. In order to further characterise the CTL response to HTLV-1, an assay to quantify specific T cell effector functions was developed. I analysed degranulation (by CD107 mobilisation) and cytokine (IFN-? and TNF-?) expression by both HTLV-1-specific CD8+ and CD4+ T cells in response to HTLV-1 Tax peptide. This study shows a trend of the frequency of degranulating CD8+ T cells to be higher in HAM/TSP patients, however at a given proviral load the rate of lysis of infected cells tend to be higher in ACs than in HAM/TSP patients. Interestingly, only the frequency of CTL degranulation correlated with the rate of killing of target cells, suggesting that CTL killing of target cells depends mainly on degranulation.