OBJECTIVE To determine age-based criteria for polycystic ovary morphology. DESIGN Cross-sectional, case-control design. SETTING Outpatient setting. SUBJECT(S) Women with polycystic ovary syndrome (PCOS) defined by hyperandrogenism and irregular menses (n = 544) and controls with regular menses and no evidence of hyperandrogenism (n = 666) participated. Parameters were tested in a second cohort of women with PCOS (n = 105) and controls (n = 32) meeting the same criteria. INTERVENTION(S) Subjects underwent a pelvic ultrasound documenting ovarian volume and maximum follicle number in a single plane. MAIN OUTCOME MEASURE(S) A receiver operating characteristic curve was constructed to determine the ovarian volume and follicle number with the best sensitivity and specificity to define PCOS for age groups at approximately 5-year intervals from age 18 to >44 years. RESULT(S) The best sensitivity and specificity were obtained using a threshold volume of 12 mL and 13 follicles for ages ≤24 years, 10 mL and 14 follicles for ages 25-29 years, 9 mL and 10 follicles for ages 30-34 years, 8 mL and 10 follicles for ages 35-39 years, 10 mL and 9 follicles for ages 40-44 years, and 6 mL and 7 follicles for ages >44 years. Data from a second cohort confirmed the need to decrease volume and follicle number with increasing age to diagnose PCOS. Polycystic ovary morphology was most accurate at predicting the PCOS diagnosis for women ages 30-39 years. CONCLUSION(S) The ovarian volume and follicle number threshold to define polycystic ovary morphology should be lowered starting at age 30.