Double disease in rheumatology: coexistence of rheumatoid arthritis and psoriatic arthritis.

Since rheumatoid arthritis (RA) and psoriatic arthritis (PA) are common diseases, there should be a group of patients in which the two entities coexist, with an estimated prevalence ranging from 0.03/10,000 to 0.15/10,000. The two entities may share clinical, analytic and radiologic features that further complicate the diagnosis. We report here a patient in whom RA and PA coexisted. The features that differentiate peripheral arthritis in RA and PA are discussed.