Pattern alopecia is the androgen-dependent thinning of hair that occurs progressively with advancing age in genetically susceptible men and women. However, it can begin as soon as the early teenage years. The process is mainly the result of miniaturization of terminal to vellus hair follicles (Fig. 1). There is also moderate loss of PSUs with time and often moderate inflammatory changes with perifollicular lymphocytic infiltration (253). The androgen dependency of pattern alopecia was initially deduced on the basis of eunuchs not suffering from male pattern hair loss unless they are given replacement testosterone (254). Pattern alopecia is generally thought to be distinct from the diffuse thinning of scalp hair associated with aging.