HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP) develops in less than 5 % of HTLV-1 carriers. It is unclear which factors trigger the development of disease. The aim of this study was to explore the influence of HLA alleles on the development of HAM/TSP. A total of 40 HTLV-1-infected individuals belonging to the Spanish HTLV-1 cohort were examined. HTLV-1 proviral load was measured by real-time polymerase chain reaction. HLA class I (A, B, C) and class II (DRB1, DQB1) alleles were genotyped using the bead array technology. Median HTLV-1 proviral load in 12 HAM/TSP patients was greater than in 28 asymptomatic carriers (637 vs. 71 copies per 10(4) peripheral blood mononuclear cells; p = 0.006). Moreover, HAM/TSP was significantly associated with HLA-B 07 and HLA-DRB1 01:01 (p = 0.039). Interestingly, individuals with these HLA alleles had greater HTLV-1 proviral load than asymptomatic carriers (p = 0.036). In summary, HLA testing should be considered in asymptomatic HTLV-1 individuals and close monitoring of HTLV-1 proviral load along with periodic neurological evaluations should be prioritized in HLA-DRB1 01:01 and HLA-B 07 carriers.