Objective: To apply and compare different classification criteria on a representative nationwide sample of psoriatic arthritis (PsA) twins and to estimate the prevalence and incidence of PsA. Methods: The study comprised three Danish nationwide twin cohorts. In 1994 37 388 Danish twin individuals and in 2002 46 418 twin individuals received a questionnaire, including questions on rheumatic diseases. Twins reporting PsA and their co-twins were classified according to the Moll and Wright and CASPAR (ClASsification criteria for Psoriatic ARthritis) criteria based on interview, clinical examination and scrutiny of medical records. Results: 228 twin individuals reported PsA and 164 (72%) participated in clinical validation. By using the Moll and Wright and CASPAR criteria, 54 and 50 cases were diagnosed with PsA respectively. The positive predictive value of self-reported PsA was 31%. According to the Moll and Wright and CASPAR criteria the prevalence was 0.15% (95% Cl: 0.13%, 0.22%) and 0.14% (95% Cl: 0.11%, 0.19%) respectively. The annual incidence rate based on new self-reported cases in 2002 was 6/100 000 person-years (95% Cl: 3/100 000 person-years, 11/100 000 person-years). Conclusions: The positive predictive value of selfreported PsA was 31%. The prevalence and incidence figures of PsA were equivalent to the previously reported occurrence in population- and hospital-based studies.