Polycystic ovary syndrome (PCOS) and hypothalamic amenorrhea (HA) are the most frequent causes of endocrine infertility, but their association is an uncommon occurrence. We report the case of a 28-year old woman suffering from infertility and amenorrhea. Her weight was normal (BMI = 19) and she had no hirsutism. She self-reported food restriction and a 10 kg weight loss 5 years ago, concomitant with the onset of amenorrhea. At the initial evaluation, the patient was considered as having HA due to food restriction. At ultrasonography, ovaries were small and multifollicular (right and left area: 2.2 and 2.5 cm(2), respectively; number of cysts 2-9 mm in diameter: 15 and 12, respectively), and no stromal hypertrophy was noted. She has been treated for 1 month by intravenous pulsatile GnRH administration. Although the doses were increased from 5 to 15 microg/pulse every 90 min, no E2 response and no follicular development were observed. Hormonal re-evaluation revealed normal levels of serum LH, FSH and androgens, and a normal LH/FSH ratio. However, a typical aspect of PCO was found at ultrasound (right and left area: 6.5 and 5.5 cm(2), respectively, and more than 15 small cysts arranged peripherally around an increased central stroma in each ovary). The treatment has been then switched to hMG, using the low dose step-up regimen and starting with 75 U/day. In the absence of response after 2 weeks, the dose was increased to 112.5 U/day and a multifollicular reaction occurred, leading to cancellation. In conclusion, we hypothesize that this patient had a "hidden" PCOS when she was hypogonadotrophic and that it developed very rapidly after restitution of a normal gonadotropin level under exogenous GnRH. This occurred despite a low insulin level, showing that hyperinsulinism is not a prerequisite for the development of PCOS in every case.