There was a 23.6-fold difference in the development rate between individuals homozygous for DRB1-GB-7-Leu at the median proviral load leading to HAM/TSP and individuals homozygous or heterozygous for DRB1-GB-7-Pro at the median proviral load who are asymptomatic carriers. Here, HLA-DRB1 is linked to tropical spastic paraparesis.