We compared the relationships among percentage of body fat (%BF) and physical activity with breast cancer (BC) and benign breast diseases (BBD) in low-income Brazilian women. A case-control study including 106 incident BC cases, 178 incident BBD cases, and 181 control women recruited from a public hospital-based screening center was conducted. Logistic regression models showed that sedentary women have a higher odds of developing BC in the age adjusted model [odds ratio (OR) = 2.39, 95% confidence interval (CI) 1.43-3.99]. After adjusting for hormonal-related risk factors and family history of breast cancer (OR = 2.94, 95% CI 1.50-5.79) and also for the % BF (OR = 2.71, 95% CI 1.36-5.37) the odds remained high. Independent of the adjustments made, %BF did not affect the odds for developing BC. When the same models were tested for women with BBD, we found a significant association with sedentary lifestyle in all models tested, with an OR = 3.03 (95% CI 1.69-5.42) in the fully adjusted model. In the same way, in the fully adjusted model %BF was significantly associated to risk for BBD (OR = 0.54, 95% CI 0.30-0.98). Similar to results found in other populations, our data suggest that physical activity is an important, independent protective factor for the risk of developing BC and BBD in low-income women from an admixed population.