OBJECTIVE: To investigate the association between body mass index (BMI) and serum AMH levels in reproductive age women with diminished ovarian reserve (DOR) diagnosed by elevated baseline FSH. DESIGN: Cross sectional observational study. MATERIALS AND METHODS: Two hundred and ninety women (ages: 21-50) were evaluated for infertility between 2007- 2010 and were included in this study. Baseline serum FSH levels were measured on day 2 or 3 of the cycle. Serum AMH levels were drawn randomly, unrelated to day of the cycle. Wilcoxon rank-sum test, multiple regression analysis and Spearman's rank correlation were used for statistical analysis. P<0.05 was considered significant. RESULTS: BMI correlated negatively with serum AMH in women with elevated baseline FSH (>10IU/L) (r= -0.27, p=0.001) but not in women with normal baseline FSH (10IU/L or less). Among women with elevated baseline FSH, AMH was 43% lower in women with high BMI (>25) compared to women with normal BMI (25 or less) (p=0.0002). This association persisted after controlling for age (p=0.03). In marked contrast, serum AMH levels were no different between high and normal BMI groups among women with normal baseline FSH. CONCLUSION: We report for the first time a negative association between BMI and serum AMH levels among women with elevated baseline FSH. We postulate that obesity has an adverse effect on ovarian reserve independent of age. Further studies are needed to understand the mechanisms of excess weight induced reduction in ovarian function.