Studies on obesity and glycemic index (GI) or glycemic load (GL) have had inconsistent results, perhaps in part because of underreporting or to heterogeneous dietary patterns across food cultures.We examined associations between body mass index (BMI) and GI or GL in a Mediterranean population, accounting for underreporting. We also constructed dietary factors related to GI and GL to better understand food patterns related to these measures.Cross-sectional data on 8195 Spanish adults aged 35-74 y were analyzed. A validated food-frequency questionnaire was used to estimate GI and GL, with glucose as the reference value. Reduced-rank regression was used to construct dietary patterns that explained variation in GI and GL. Multivariate linear regression was used to estimate associations between BMI and GI, GL, and their respective diet factors with and without adjusting for energy, which may lie on the causal pathway between glycemic quality and obesity. Effects of excluding underreporters (ratio of energy intake:basal metabolic rate < 1.20) were examined.Food patterns underlying high GI differed substantially from those of high GL, with fruits, vegetables, and legumes related positively to GL but negatively to GI. After excluding underreporters, GL was negatively associated with BMI, adjusting for energy. GI was not associated with BMI in any model.After adjusting for energy, GL was associated with reduced BMI in this Mediterranean population. Underreporting did not explain this inverse relation, which was observed among subjects with plausible intakes.