The analysis of population-specific human leukocyte antigen (HLA) haplotypes has provided evidence that susceptibility to psoriasis is linked to the class I and II major histocompatibility complex on human chromosome 6. In addition, these studies show that psoriasis consists of two distinct disease subtypes (type I and type II), which differ in age of onset and in the frequency of HLA. In type I (early-onset) psoriasis, Cw6, B57, and DR7 are strongly increased, whereas in type II (late-onset) psoriasis, HLA-Cw2 is overrepresented. It has also been proposed that HLA haplotypes extended by class III play a role in the genetics of this disease. Moreover, studies of affected families indicate that other disease susceptibility loci may also be involved. Likely candidates for additional susceptibility genes are located at chromosomes 1, 6, and 17, and microsatellite markers over the whole genome have been used to identify susceptibility genes. Two years ago linkage to the distal part of chromosome 17 was published. However, this linkage could not be confirmed by other groups with comparable or enlarged numbers of psoriatic family members investigated. Recently, an investigation presenting an area of chromosome 4 as a susceptibility locus for psoriasis was published. According to our knowledge today, psoriasis is a polygenetically inherited disease. Furthermore from twin studies it is known that environmental factors play a significant role in the onset or recurrence of the disease. (J Am Acad Dermatol 1997; 37:S1–S11.)