Background Previous twin studies have shown greater concordance rates for psoriasis in MZ than in DZ twins, and heritability estimates between 66% and 90%. This supports a genetic influence on psoriasis, but also highlights the fact that genes are not the only explanation for the disease. Objectives To study the concordance of psoriasis in a population-based twin sample. Methods Data on psoriasis in 10 725 twin pairs, aged 20-71 years, from the Danish Twin Registry was collected via a questionnaire survey. The concordance and heritability of psoriasis were estimated. Results In total, 4·1% of the men and 4·2% of the women had a lifetime history of psoriasis. The proband-wise concordance for psoriasis was larger in monozygotic than in dizygotic twins, 0·33 vs. 0·17. Genetic factors explained 68% (60-75%) of the variation in the susceptibility to psoriasis, whereas the rest of the variation was explained by nonshared environmental factors. Conclusion The results confirm that psoriasis is a complex multifactorial disease controlled by both exogenous and endogenous factors. What's already known about this topic? A few twin studies have previously been conducted in patients with psoriasis suggesting a genetic contribution to psoriasis. The concordance rate and heritability have been estimated but not on as large a population as in the current study. What does this study add? This is the largest twin study on psoriasis to date, using the unique Danish Twin Registry involving 10 725 twin pairs, making it possible to study the familial resemblance of psoriasis and achieve accurate concordance and heritability estimates of psoriasis.