Background: Psoriatic arthropathy is a seronegative arthropathy seen in some patients with psoriasis, a self-limited and genetically determined skin disease. Objective: To determine the prevalence and risk factors of arthropathy in patients with psoriasis. Patients and Methods: In a prospective, cross-sectional study, psoriatic patients referred to Razi Skin Hospital in Tehran, Iran in 2002 were evaluated for the presence of arthropathy, and were referred to a rheumatologist for confirmation of arthropathy. The relationship between and nail involvement were evaluated using chi-square and t tests. Results: Among 320 patients with psoriasis, 29 (9.1%) had joint diseases. The most common was asymmetric polyarthritis. Patients with joint diseases had more nail involvement (96.5% vs. 73.1%, P<0.05) and higher mean PASI scores (24.33±10.35 vs. 10.70±8.44, P<0.05) than those without arthropathy, but the age and sex distribution of two groups were not significantly different. Conclusion: The prevalence of joint diseases in psoriatic patients in this study was similar to other reports. Nail involvement and more extensive disease are risk factors for joint diseases in these patients.