To identify diagnostic markers of psoriasis vulgaris and psoriatic arthritis, autoantibodies in sera from patients with psoriasis were screened by two-dimensional immunoblotting (2D-IB). We previously found 22 autoantigens each in psoriasis sera. In this study, serum levels of valosin-containing protein (VCP) in patients, one of the identified autoantigens in psoriasis patients, were studied by reverse-phase protein array analysis using sera from patients with psoriasis and healthy controls. Serum levels of VCP were significantly higher in patients with psoriatic arthritis than in controls or patients with psoriasis vulgaris (P<0.05 each). The area under the curve for VCP between patients with psoriatic arthritis and controls or psoriatic vulgaris patients was 0.88 and 0.77, respectively. The serum level of VCP was weakly correlated with the disease activity of psoriasis patients and psoriasis area severity index score. These data suggest that VCP is a novel and differential sero-diagnostic marker of psoriatic arthritis.