OBJECTIVE: To evaluate whether some ultrasound parameters of ovarian morphology can discriminate between control women and patients with polycystic ovary syndrome (PCOS). DESIGN: Retrospective data analysis. SETTING: Volunteers women in an academic research environment. PATIENT(S): Eighty amenorrheic or oligomenorrheic women and 30 normal ovulatory control participants. INTERVENTION(S): None. MAIN OUTCOME MEASURE(S): We evaluated ovarian volume, area, stroma, and the stroma/total area (S/A) ratio by use of transvaginal pelvic ultrasound; and we assayed serum levels of gonadotropin, androgen, and estradiol during the early follicular phase (days 2 to 5) of the menstrual cycle in regularly cycling controls and on a random day in amenorrheic patients. RESULT(S): Patients with PCOS showed significantly higher ovarian volume, area, stroma, and mean S/A ratio when compared to multifollicular and control groups. Cut-off values have been defined for ovarian volume (13.21 mL), area (7.00 cm2), stroma (1.95 cm2), and S/A ratio (0.34). The sensitivity for PCOS diagnosis was 21%, 4%, 62%, and 100%, respectively. The S/A ratio showed the most significant correlation with the androgen levels. CONCLUSION(S): The evaluation of the S/A ratio can differentiate between PCOS and control or multifollicular women with both a sensitivity and a specificity of 100%. Furthermore, this ultrasound parameter is strictly related to hormonal milieu and to anthropometric characteristics.