A surprisingly high association between an HLA-DR locus antigen and narcolepsy was revealed in the Japanese. All 40 Japanese patients with narcolepsy (22 males and 18 females) were confirmed to be DR2 positive. However, the DR2 of the patients was found to associate negatively with Bw52, whereas Bw52-DR2 is the commonest haplotype in normal Japanese population. These data suggested that a "disease" allele predisposing to narcolepsy was inherited with relatively rare haplotypes with DR2 in the Japanese.