Polycystic ovary syndrome (PCOS) is a common endocrine disorder in reproductive-age women. Hyperandrogenism, chronic anovulation, and infertility are the main features of this heterogeneous condition. The diagnosis of PCOS is based on a combination of clinical, biological, and ultrasound criteria that have been used variably to define PCOS. The usual clinical presentation of PCOS in Asia is slightly different from that in the United States and Europe, with less frequently encountered cases of hyperandrogenism. Moreover, non-obese PCOS is typical in Asian women. The Japanese Society of Obstetrics and Gynecology has recently proposed new, revised diagnostic criteria. Growth arrest of ovarian follicles in the non-obese PCOS is assumed to be associated with an abnormal endocrine environment involving chronically elevated LH. In vitro studies currently demonstrate that LH promotes follicular growth during preantral-early antral transition via the increased synthesis and growth promoting action of androgen. However, chronic LH stimulation impairs FSH-dependent antral follicle growth by suppressing FSH receptor expression in granulosa cells, via the modulation of intraovarian regulators. Therefore, the adverse effect(s) of chronically elevated LH on the theca cell/androgen system should also be considered for improved care of non-obese PCOS patients.