The interval between the appearance of the symptoms of psoriasis and/or arthritis and the setting up the diagnosis of the psoriatic arthritis was studied in 215 patients suffering from definite psoriatic arthritis. About 2.3 years were over until the setting-up of the diagnosis in these 30 patients whose psoriasis and arthritis began simultaneously. The interval to set up the diagnosis of psoriatic arthritis was 5.4 years in average if the psoriasis itself was the first sign and it was 8.6 years in case the arthritis preceded psoriasis. The symptoms promoting to set up the right diagnosis were in order to frequency as follows: appearance of psoriasis, sausage digits, distal interphalangeal involvements, nail changes and transformation the monoarthritis into asymmetrical oligoarticular or polyarticular form. The difficulty of the differential diagnosis was studied. 15 different previous false diagnoses were enumerated the rate of which was the highest (67.3%) in the group starting with arthritis. The authors call the attention to the importance of looking for psoriatic skin and nail changes in every nonclassified arthritic patient in the interest of an early diagnosis and right therapy in course of the follow-up.