Carcinoma in situ of the vulva is, at present, a poorly understood entity and little is known of its etiology and natural history. A case is reported of carcinoma in situ of the vulva in identical twins related to chronic contact of the vulvar skin with arsenical insecticides during farming operations. This report emphasizes the role of hereditary predisposition in the development of cancer and serves to refocus attention on arsenic, a common constituent of insecticides and detergents, as an etiologic agent in carcinomas of the skin.