SINCE we have but meager knowledge of the factors influencing the growth of hair, and although the failure of hair growth creates greater popular interest, examples of hypertrichosis from any cause should be recorded in order to enhance our general information on the hair problem. With this in mind we are presenting a series of 13 patients in whom striking acquired localized hypertrichosis occurred as a result of prolonged, frequently repeated self-biting in the mentally deficient. Hypertrichosis, excessive or abnormal growth of hair from any cause, may have local or general distribution and may be congenital or acquired. The most celebrated of the congenital hypertrichoses is the Russian "dog-faced" boy who was described by Jackson 1 in 1886. In this congenital type the affected person is born with generalized hypertrichosis. The excessive hair continues to grow after birth and increases in length and coarseness. Partial congenital hypertrichosis is fairly