Abstract Background: It has long been established that obesity plays a positive role against osteoporosis (OP) and low-impact fractures (Fx). However, more recent data has shown higher fracture risk in obese individuals. The aim of this study was to investigate the association between BMI, particularly obesity, OP and low-impact Fx in Brazilian women, as well as to evaluate the SAPORI (Sao Paulo Osteoporosis Risk Index) tool performance to identify low BMD according BMI category. Methods: A total of 6182 women aged over 40 years were included in this cross-sectional analysis using data from two large Brazilian studies. All participants performed hip and spine bone mineral density (BMD) measurements and answered a detailed questionnaire about the presence of clinical risk factors (CRFs) related to low BMD and risk fractures. The World Health Organization (WHO) criteria were used to define obesity. Results: Age-adjusted osteoporosis prevalence was 20.8, 33.6, 47 and 67.1% in obese, overweight, normal and underweight category, respectively. Obesity was present in 29,6% (1.830 women) in the study population and the likelihood of osteoporosis and low-impact Fx compared to a normal BMI in this subgroup was of 0.24 (95% CI 0.20– 0.28; p < 0.001) and of 1.68 (95% CI 1.35–2.11; p < 0.001), respectively. However, the hip Fx likelihood was lower in obese compared with non-obese women (OR = 0.44; 95% CI 0.20–0.97). Using an originally validated cut-off, the SAPORI tool sensitivity was significantly hampered in overweight and obese women although the accuracy had remained suitable because of increasing in specificity. Conclusions: The osteoporosis prevalence reduced as BMI increased and obesity was associated with low-impact Fx, regardless of the BMD measurements. Moreover, the SAPORI performance was impaired in obese women.