The frequency of alopecia areata and observed patterns of heritability are in keeping with a polygenic inheritance model but the genetics of alopecia areata is still poorly understood. The role of environmental factors in triggering disease initiation or exacerbation remains almost entirely speculative. Using the candidate gene approach, three susceptibility/severity factors have been identified. HLA alleles were the first to show a strong association with alopecia areata and some DQB and DR alleles have been demonstrated to confer a high risk for disease by both case-control and family-based studies. Interleukin (IL)-1 cluster genes, mainly the IL-1 receptor antagonist, show a strong association with disease severity in alopecia areata and a number of other autoimmune and inflammatory diseases. Finally, the association of alopecia areata with Down's syndrome, the high frequency of alopecia areata in autoimmune polyglandular syndrome type I due to mutations of the autoimmune regulator (AIRE) gene on chromosome 21q22.3 and the finding of association with MX1, another gene in the Down's syndrome region of chromosome 21 indicate this area of the genome as a promising target for future-family based investigations. The role of individual genes of the MHC, IL-1 cluster or chromosome 21q22.3 is difficult to establish and further genetic and functional investigations are needed to confirm their involvement in the pathogenesis of alopecia areata.