The relative importance of genetic factors in the origin, age at onset, clinical type, course, and severity of psoriasis was evaluated on the basis of an unbiased sample of twins, ie, the Danish Twin Register, which covers the total population of twins born in Denmark. All verified and probable cases of psoriasis in twins, born 1891 through 1920, were ascertained. Results are presented of an examination of all members of index pairs in which both partners were alive on a certain date. Fourteen monozygotic and 22 dizygotic, like-sexed pairs were found to include at least one partner with unquestionable psoriasis. Zygosity determination was mainly based on extensive serological examinations. The analyses show that the manifestation of psoriasis depends almost exclusively on the presence of the specific genotype. The age at onset, clinical type, course, and severity are also mainly determined by the genetic constitution. Association with certain HLA antigens of the B series has been confirmed, but the fact that many of the twins (including several of the concordant monozygotic pairs) possess neither of these antigens shows the corresponding genes to be important, but not decisive, elements in the predisposition. We conclude that psoriasis is a genetically determined disorder that may, to a limited extent, be modified by environmental influences.