Psoriatic arthritis (PsA) is an inflammatory arthritis that may affect as many as 30% of patients with psoriasis (Ps). Genetic factors play an important role in the susceptibility to and the expression of PsA. The objective of this study was to identify whether haplotype sharing among affected sibling pairs of individuals with PsA is increased compared with unaffected sibling pairs. We collected 182 sibling pairs of probands affected with PsA. Extracted genomic DNA was amplified in polymerase chain reactions using locus specific primers homologous to nucleotide sequences for each of the HLA-A, -B, -C, -DR, and -DQ loci. Polymerase chain reaction amplicons were identified by reverse line blot assay using sequence-specific oligonucleotide probes. Evidence for excessive haplotype sharing was examined through Green and Woodrow's test. Results indicate that of the 182 sibling pairs, 46 were affected by PsA, 48 by Ps, and 88 were unaffected. The sharing of 2, 1, and 0 haplotypes for the PsA affected sibling pairs was 14, 27, and 5, respectively (p = 0.04); whereas the haplotype sharing for the Ps affected sibling pairs was 12, 26, and 10, respectively (p = 0.38). In conclusion, the human leukocyte antigen region on chromosome 6p is implicated as one of the candidate regions in PsA.