HTLV-1 associated myelopathy/tropical spastic paraparesis (HAM/TSP) is one outcome of Human T-cell lymphotropic virus type 1 (HTLV-1) infection. It remains unknown why the majority of infected people remain healthy whereas only approximately 2-3% develop disease. Recently, heterozygous state of CD45 exon 4 mutation (C77C wild type and C77G mutant) was reported to be associated with development of multiple sclerosis in German patients and increased susceptibility to HIV-1 infection in the United Kingdom. To investigate whether this mutation is associated with the development of HAM/TSP, we studied a group of 164 HAM/TSP patients and 108 asymptomatic HTLV-1 carriers in Kagoshima (HTLV-1 endemic area in Southern Japan) by using PCR-RFLP and subsequent direct sequencing analysis. All 272 subjects showed homozygosity in the CD45 exon 4, suggesting that this mutation is absent or very rare in Japanese population.