Analysis of the presence of psoriasis in all first-degree relatives of psoriatic twin probands, 38 monozygotic (MZ) and 24 dizygotic (DZ), has been performed in order to clarify if genetic heterogeneity might be present (the twin-family method). The probands were derived from a population based sample of like-sexed twin pairs. An almost identical frequency of psoriasis in parents, siblings and children, with no sex difference, was found. By comparisons of empirical risk figures for psoriasis in first-degree relatives of concordant as compared with discordant MZ probands and HLA-B 13 and/or HLA-B 17 positive MZ probands compared with MZ probands lacking these antigens, no clue to the presence of genetic heterogeneity was found. An almost identical risk in co-twins and ordinary siblings of DZ-probands was found. The data were incompatible with autosomal recessive or X-linked inheritance, but not incompatible with autosomal dominant inheritance with reduced penetrance or with multifactorial inheritance.