Genetic association of some immune-mediated human uveitic diseases with histocompatibility antigens, ethnic origin, familial background, or gender have suggested the presence of a hereditary component in susceptibility to uveitis. Uveitis is a genetically complex disease, in which genes and environment contribute to the phenotype appearance. In complex traits, genotypes of particular sets of genes, together with environmental factors, alter the probability that an individual will express the characteristic, although each individual factor is typically insufficient to cause the disease. The main susceptibility genes for clinical and experimental uveitis seem to be located within the major histocompatibility complex (MHC) region, but genes possibly regulating responses to lymphokines, hypothalamic-adrenal-pituitary axis hormones, vascular effects, and possibly T cell repertoire and other pathways play a role to determine "permissiveness" or "nonpermissiveness" to the disease.