Hirsutism may arise from two different causes: an increased production of active androgens by the adrenals and/or an increased utilization of circulating androgens by the target cells of the skin i.e. idiopathic hirsutism. Plasma testosterone and androstenedione levels are the best indexes of androgen production. Testosterone 5 alpha-reductase capacity in the skin and urinary 3 alpha-androstanediol reflect androgen utilization. In most hirsutisms, the only plasma testosterone level give sufficient information: it is normal with normal ovulatory cycles in idiopathic hirsutism, it is slightly elevated in ovarian dystrophy, the most frequent causes of hirsutism. More sophisticated investigations are only needed when testosterone levels appear to be over 1.50 ng/ml. In these cases, one could expect an uncommon etiology such as virilizing tumor on a delayed onset of congenital adrenal hyperplasia.