One hundred and eighty patients with psoriasis have been studied in the Neapolitan area to find the prevalence of arthritis in psoriasis. Wright and Moll's criteria for the diagnosis of arthritis were applied. Of 180 psoriatic patients, 62 exhibited arthritis (34.4% of total cases) According to Moll and Wright's five broad clinical forms of arthritis, the following distribution was found: polyarticular in 38.7% of arthritic patients, mono-oligoarticular in 16.1%, distal interphalangeal in 7.5%, deforming or mutilans in 2.3% and spondylitic and/or sacro-iliitic in 20.9%. In 14.5% of arthritic patients an overlap of the spondylitic form and peripheral involvement was also found. Stratification by age of patient populations shows that cutaneous pathology has a homogeneous distribution in the various age decades, while joint symptoms are seen maximally in the sixth decade and are absent in the first two decades. The relationship between the onset of skin lesions and joint pathology demonstrates that skin lesions preceded arthritis in 64.5% of the cases, whilst arthritis antedated psoriasis in only 19.35%. In 16.1% of cases psoriasis and arthritis began almost simultaneously. Nail changes were present in 63% of arthritic patients and in 37% of psoriatic subjects without arthritis. Furthermore, in 88% of arthritic patients in whom arthritis preceded skin lesions, nail changes antedated the onset of clinically apparent psoriasis. Extra-articular features were not found. This absence might be associated with the usual seronegativity for rheumatoid factor in psoriatic arthritis, also confirmed in this survey.