BACKGROUND: This study revisited the ultrasonographic diagnostic criteria of polycystic ovary syndrome (PCOS) and studied the relationship between the major hormonal and metabolic features of PCOS and the follicle number per ovary (FNPO). METHODS: This prospective study included 214 women with PCOS compared with 112 women with normal ovaries. Main clinical, biological and ultrasonographic markers of PCOS were assessed during the early follicular phase. RESULTS: The mean FNPO of follicles 2±5 mm

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... in size was signi®cantly higher in polycystic ovaries than in controls, while it was similar within the 6±9 mm range. Setting the threshold at 12 for the 2±9 mm FNPO offered the best compromise between speci®city (99%) and sensitivity (75%). Within the 2±5 mm follicular range, we found signi®cant positive relationships between the FNPO and androgens. The FNPO within the 6±9 mm range was signi®cantly and negatively related to body mass index and fasting insulin serum level. CONCLUSIONS: We propose to modify the de®nition of polycystic ovaries by adding the presence of b12 follicles measuring 2±9 mm in diameter (mean of both ovaries). Also, our ®ndings strengthen the hypothesis that the intraovarian hyperandrogenism promotes excessive early follicular growth and that further progression cannot proceed normally because of hyperinsulinism and/or other metabolic in¯uence linked to obesity.