Psoriatic arthritis (PsA) is an inflammatory arthritis associated with psoriasis. There are no generally accepted diagnostic criteria for PsA. Indeed, the diagnosis of this inflammatory arthritis is made by exclusion of other possible diseases and based upon immunologic, radiologic, and clinical features which are consistent with the diagnosis. Inflammatory arthritis in a patient with psoriasis can be an important clue for the diagnosis of PsA, but the possibility for diagnosis of other inflammatory arthritides ever remains. Herein we report a case of a female patient who was not diagnosed with PsA, but with rheumatoid arthritis, even though she had psoriasis. (Korean J Intern Med 2010;25:224-226)