The regulation of human hair growth is one of the most mysterious of biologic events. Genetic factors are responsible for the density, length, color and texture of hair. Heredity disorders can affect either hair alone, hair and other ectodermal structures, or can be pansystemic. This paper considers all heredity disorders which affect human hair growth. These disorders are divided into three categories: 1) the dysmorphic syndromes; 2) the modifier gene abnormalities and 3) the single major gene abnormalities. The disorders in these categories are listed, along with the mode of inheritance, cause, hair phenotype and the extent of involvement. The utilitarian function of the study of human hair growth is illustrated. Insight into problems of differentiation can be most rapidly expanded using this approach.