Susceptibility to a clinically significant drug hypersensitivity syndrome associated with abacavir use seems to have a strong genetic component. We have previously shown that the presence ofHLA-B*5701strongly predicts abacavir hypersensitivity and have identified a potential susceptibility locus within a 300-kb region between theMEGT1andC4A6loci in the central MHC. We now report the results of fine recombinant genetic mapping in an expanded patient population of 248 consecutive, fully ascertained, abacavir-exposed individuals in the Western Australian HIV Cohort Study, in which 18 cases of definite abacavir hypersensitivity (7.3%) and 230 tolerant controls were identified. Haplotype mapping within patients with allelic markers of the 57.1 ancestral haplotype suggests a susceptibility locus within the 14-kbHsp70gene cluster.HLA-B*5701was present in 94.4% of hypersensitive cases compared with 1.7% of controls (odds ratio, 960;P< 0.00001). A haplotypic nonsynonymous polymorphism ofHsp70-Hom(HspA1L, resulting from the substitution of residue M493T in the peptide-binding subunit) was found in combination withHLA-B*5701in 94.4% of hypersensitive cases and 0.4% of controls (odds ratio, 3,893;P< 0.00001). Individuals with abacavir hypersensitivity demonstrated increased monocyte tumor necrosis factor expression in response toex vivoabacavir stimulation, which was abrogated with CD8+T cell depletion. These data indicate that the concurrence ofHLA-B*5701andHsp70-Hom M493Talleles is necessary for the development of abacavir hypersensitivity, which is likely to be mediated by anHLA-B*5701-restricted immune response to abacavir.