Six hundred and forty-seven patients with psoriasis were studied in order to define prevalence and clinical features of psoriasis and psoriatic arthritis. After medical history review all patients were observed by a dermatologist and a rheumatologist. Successive laboratory tests and radiological and scintiscan examinations of joints were also performed. Diagnosis of arthritis was made according to Wright and Moll. In our district, the prevalence of psoriasis in respect to other dermatological diseases was 1.86%. Vulgaris pattern occurred in 85% of the total cases while eruptive was found in 10.5%. Erythrodermic and pustular forms were uncommon, occurring in 2.47% and in 1.23%, respectively. In 84.8% of the total cases, psoriasis onset was clinically evident while in 10.8% it affected non-evident cutaneous sites. Onycopathic onset occurred in 4.3%. Altogether 138 psoriatic patients exhibited arthritis and spondylitic and polyarticular patterns were very common (occurring in 42.7% and 33.3%, respectively). Psoriasis antedated arthritis in 68.4% of the cases but followed it in 21%. Synchronous onset occurred in 10.8%. The data obtained strengthen those of our previous reports. In particular, the marked articular involvement of psoriatic patients is confirmed. Moreover, our results, when compared with data of other authors, show a different distribution among the arthritic subtypes.