The purpose of this study was to determine whether the inheritance of psoriasis is HLA linked. We studied 12 families with at least two siblings with psoriasis. We used a sib pair analysis comparing the HLA haplotypes in affected siblings of these families. Haplotype sharing was analyzed in 15 sib pairs; expected frequencies of haplotype sharing were estimated as 25% for HLA identity and 50% for haplotype identity. All sib pairs shared at least one haplotype and 13 of the 15 were HLA identical, compared with the expected frequency of four. These results suggest that at least one, and probably more, HLA-linked genes are implicated in the development of psoriasis.