OBJECTIVE: To evaluate the role of mean ovarian volume (MOV) in the diagnosis of polycystic ovary syndrome (PCOS) during adolescence, and its relationship with metabolic and endocrine parameters. DESIGN: Observational study PATIENTS: A total of 134 young girls, including 86 adolescents with PCOS and 48 controls, were studied. MEASUREMENTS: During the early follicular phase a pelvic ultrasound examination was performed to measure the ovarian volume of both ovaries and to calculate the MOV. All subjects underwent hormonal assessment and an ultrasound examination. PCOS subjects were submitted to an oral glucose tolerance test. The homeostasis model assessment for insulin resistance (HOMA-IR) and several insulin resistance indexes were also determined. RESULTS: Androgens, free androgen index (FAI), LH and insulin resistance indexes were higher in the PCOS group. MOV was significantly different between the two groups: control group 4.6± 1.9 cm(3) , adolescent PCOS group 9.6 ± 4.4 cm(3.) The MOV threshold of 5.596 cm(3) offered the best compromise between sensitivity and specificity based on the characteristics of the operating receiver curve analysis. Therefore, an ovarian volume higher than 5.6 increased the risk of PCOS by about 15 times (OR 16.25 IC 95% 6.3-41.3). In adolescent PCOS girls the ovarian volume was significantly associated with circulating testosterone and insulin, and indices of insulin resistance. CONCLUSIONS: During early adolescence MOV evaluation may offer an effective means to screen and follow up young girls with irregular cycles in order to prevent the long term metabolic disturbances of the polycystic ovary syndrome. © 2012 Blackwell Publishing Ltd.