An overview of the partial 3 beta-hydroxysteroid dehydrogenase deficiency is presented. The classical, congenital or early postnatal form is characterized by a salt-losing syndrome and/or ambiguous genitalia. The late-onset forms, only recognized for the last ten years and diagnosed with an increasing frequency, are to be systematically suspected in the presence of clinical hyperandrogenism with or without oligomenorrhea. This deficit, involved in both adrenal and gonadal tissues, seems to be transmitted by an autosomal recessive gene. An ovarian 3 beta-hydroxysteroid dehydrogenase deficit can be a primary cause of some cases of polycystic ovary syndrome and the relations with this affection are disputed. The increased ratios of delta 5 steroids/delta 4 steroids ensure the diagnostic conviction while the elevated ratio of 17-hydroxy-pregnenolone/17-hydroxyprogesterone and the normal ratio of 11-desoxycortisol/cortisol allow to eliminate the possibility of a 21-hydroxylase or 11 beta-hydroxylase deficiency, respectively. The treatment is based above all on the glucocorticoid utilization, which can lead to the return of menses and the ovulatory function, but the cutaneous symptoms of hyperandrogenism will be better controlled by cyproterone acetate out of situations of stress.