Objective: To determine whether magnetic resonance (MR) imaging can serve as a useful investigational tool in the assessment of the polycystic ovary as compared with transabdominal ultrasound (US) for obese adolescents with polycystic ovary syndrome (PCOS). Design: Prospective observational study. Setting: Tertiary university hospital. Patient(s): Eleven obese adolescents with PCOS. Intervention(s): Pelvic MR and US imaging and blood sampling. Main Outcome Measure(s): Total ovarian volume and follicle count; mean follicle count per longitudinal cross-section; stromal area; biochemical correlations with measured ovarian parameters. Result(s): With MR, the mean (±SE) total follicle count (21.9 ±1.3) was significantly greater than that observed with US (5.5 ±1.7) and significantly correlated with total ovarian volume. Two-dimensional cross-sectional analysis of the ovary by MR revealed a significantly greater mean follicle count (12.1 ±0.8) compared with the results obtained by US (3.0 ±0.5). The stromal area (173.3 ±25.1 mm2) was approximately 35%of the total ovarian surface area. No correlations were observed between biochemical indices and measured ovarian parameters. Conclusion(s): In contrast to US, MR provides vastly greater delineation of the structural components of the ovary in obese girls with PCOS and thus can serve as an excellent investigational technique to assess the morphological transformation of the adolescent ovary. Objective: To determine whether magnetic resonance (MR) imaging can serve as a useful investigational tool in the assessment of the polycystic ovary as compared with transabdominal ultrasound (US) for obese adolescents with polycystic ovary syndrome (PCOS). Design: Prospective observational study. Intervention (s): Pelvic MR and US imaging and blood sampling. Main Outcome Measure (s): Total ovarian volume and follicle count; mean follicle count per longitudinal Results (s): With MR, the mean (± SE) total follicle count (21.9 ± 1.3) was significantly greater than that observed with US (5.5 ± 1.7) and Two-dimensional cross-sectional analysis of the ovary by MR revealed a significant greater mean follicle count (12.1 ± 0.8) compared with the results obtained by US (3. Conclusions (s): In contrast to US, MR provides vastly greater delineation of the structural components of the ovary in obese girls with PCOS and thus can serve as an excellent investigational technique to assess the morphological transformation of the adolescent ovary.