The variation in expression of psoriasis, in individuals with identical genetic constitutions, i.e. monozygotic twins, has been studied in a population-based sample of monozygotic twins in the Danish Twin Register. All verified and probable cases of psoriasis in twins, born between 1891 and 1930 inclusive, were ascertained. Results are presented of an examination of all members of index pairs in which both partners were alive. The zygosity determination was based in 94% of the pairs on very extensive serological examinations. Thirty-two monozygotic pairs were found to include at least one partner with unquestionable psoriasis (18 concordant, 14 discordant). The analyses give firm evidence of the contribution of genetic factors to the manifestation, age at onset, clinical type, course, and severity of psoriasis. A close association between psoriasis and HLA-B 13 an B 17 was found in both discordant and concordant pairs. No difference was found between partners from discordant MZ-pairs with regard to infections or marked 'stress' conditions.