OBJECTIVE To propose a theory for the pathogenesis of the polycystic ovary syndrome that explains the endocrinologic abnormalities of the syndrome and provides a rational approach to therapy. METHODS An English-language literature search using MEDLINE (1975 to 1988) and extensive bibliographic reviews of identified articles. METHODS We reviewed the literature and selected 169 articles considered most relevant for the definition of the endocrinologic abnormalities, elucidation of pathogenic mechanisms, or delineation of therapeutic interventions. METHODS The authors independently assessed study quality and data concerning endocrinologic abnormalities, pathogenic mechanisms, and therapy of the polycystic ovary syndrome. RESULTS The polycystic ovary syndrome may be best defined as functional, gonadotropin-dependent ovarian hyperandrogenism. The polycystic ovary syndrome results when a primary defect increases one of three variables: the ratio of the serum concentrations of luteinizing hormone to follicle stimulating hormone, the ratio of the intraovarian concentrations of androgen to estrogen, or follicular atresia. An increase in one of these variables can then induce successive abnormalities in one or more of the remaining two variables in a to-and-fro manner. CONCLUSIONS Evidence suggests that several causes exist, each of which can produce the clinical syndrome by a to-and-fro interaction among pathogenic mechanisms. Treatment objectives for the syndrome include amelioration of hirsutism, induction of ovulation, and weight reduction.