Objective: A nationwide unselected twin population to estimate the relative importance of genetic and environmental effectors in the aetiopathogenesis of psoriatic arthritis (PsA). Methods: The study comprised three Danish nationwide twin cohorts. In 1994 and 2002 a total of 37 388 and 46 418 Danish twin individuals respectively were asked by questionnaire if they had PsA. Twins reporting PsA were invited to participate in a clinical examination. Patients were classified according to the Moll and Wright and the CASPAR (ClASsification criteria for Psoriatic ARthritis) criteria. Heritability was estimated by probandwise concordance rates and variance component analysis. Results: 228 twin individuals reported PsA. Following diagnostic validation in 164 (70%), 50 probands were diagnosed with PsA according to the Moll and Wright criteria. Five of their co-twins were either dead, had emigrated, or did not participate in the twin study and nine did not respond, resulting in 36 complete pairs. A total of one of 10 monozygotic pairs and one of 26 dizygotic pairs were concordant for PsA, yielding a 6.2% difference in proportions (95% Cl: -11%, 37%). Five of 10 monozygotic pairs and four of 26 dizygotic pairs were concordant for psoriatic skin disease implying a 35% difference (95% Cl: 2%, 60%, p < 0.05). Conclusions: This first twin study on PsA confirms that genes are important in the causation of psoriatic skin disease. Despite the limited statistical power, the almost identical concordance rates for PsA in monozygotic and dizygotic twins stresses the importance of the continued search for non-genetic effectors in PsA.