Background: Antiperinuclear factor (APF) is an autoantibody directed against (pro)-filaggrin molecules. Objective: We evaluated whether APF determination is useful for the diagnosis of psoriatic arthritis (PA). Methods: We determined APF titers in sera from patients with PA (n = 76), psoriasis vulgaris (n = 38), noninflammatory rheumatic diseases (NIRDs, n = 119), rheumatoid arthritis (RA, n = 159) both with negative (n = 36) and positive (n = 123) rheumatoid factor (RF) tests, as well as from 204 healthy controls. We used an indirect immunofluorescence test on epithelial cells from human buccal mucosa as a substrate. Results: In patients with PA, 7.9% of the serum samples were APF-positive. The incidence was greater than in healthy controls (1.0%; P < .01), similar to those with uncomplicated psoriasis (2.6%; P = NS) and NIRDs (4.0%; P = NS), and lower than in RF-negative (52.7%; P < .001) and RF-positive (90.2%; P < .001) patients with RA. Three APF-positive patients with PA had symmetric joint involvement and 3 had pustulotic arthroosteitis. Conclusion: The APF test may be useful in differentiating PA from RA, and APF may be specific for two PA subgroups.(J Am Acad Dermatol 1999;40:910-3.)