Late-onset adrenal enzymatic deficiencies are a common cause of hirsutism. They can produce a PCO syndrome-like clinical course, underscoring the multi-causality of this condition. We report a 18 year old woman with secondary amenorrhea and hirsutism, whose serum levels of testosterone and DHEA-S were 175 ng/dl and 7.3 micrograms/ml and the LH/FSH ratio was 5.9. Dexamethasone decreased testosterone and DHEA-S levels, while the ACTH stimulation test increased the 17 hydroxypregnenolone serum concentration and the 17 hydroxypregnenolone/17 hydroxyprogesterone ratio to values considered diagnostic of 3 beta hydroxysteroid dehydrogenase deficiency.