Background A wide range in the prevalence (<0.01–0.25%) and incidence (0.5–23.1/100 000) of psoriatic arthritis (PsA) is reported. The main objective of this study was to examine the prevalence and incidence of PsA in central Norway.

Method The patients were recruited from the Nord-Trøndelag Health Study 3, a population study carried out in 2006–2008. All 94 194 inhabitants aged >20 years were invited and 50 806 (54%) responded. The study consisted of a questionnaire (Q1) and a brief medical examination. Q1 included questions if the persons suffered from psoriasis, rheumatoid arthritis (RA) or ankylosing spondylitis (AS). Patients with self-reported psoriasis further answered a specific questionnaire on psoriasis including a questionnaire concerning PsA. In order to identify patients with PsA we used the following criteria: Persons reporting they had or may have PsA; persons answering that they had psoriasis and RA; and persons answering that they had psoriasis and AS. Using this approach, 1278 patients were identified. Hospital files were evaluated by a rheumatologist according to a predefined protocol to verify the diagnosis of PsA.

Results 338 patients, 144 men and 194 women, were verified to have PsA. The prevalence of PsA was 6.7 (95% CI 5.9 to 7.4) per 1000 inhabitants >20 years with no significant difference between men and women. In the 9-year period of 2000–2008, a total of 188 patients were diagnosed with PsA, which give an incidence rate of 41.3/100 000 (35.8–47.6).

Conclusions The prevalence of PsA in central Norway appears to be higher than previously reported. The reason for this is unknown and may include environmental factors, life style factors and genetic differences.