HLA antigens were determined in 14 Finnish families with psoriatic members. The pedigrees of all families are presented. The results showed a clear association between HLA B13 and inheritable psoriasis, for all the 14 HLA analysed psoriatic patients in four families had B13. In one family all five psoriatic patients had the haplotype HLA-A10, Bw17; in two other families the association between Bw17 and psoriasis was less obvious. In three families six of the eight children with the haplotype A1, Bw37 had psoriasis. In all these families one parent had psoriasis or psoriatic relatives and the other parent contributed A1, Bw37. It is suggested that Bw37, in association with other genetic factors, indicates a high risk of developing psoriasis. In one family both the father with psoriatic arthritis and the son with post-urethritic Reiter's disease had A2, B27. This haplotype was also possibly associated with psoriatic arthritis in two families.