Background: Environmental ionizing radiation has been associated with increased cancer risk by several studies.  The Brazilian city of Poços de Caldas, MG, seats on a huge deposit of uranium, which was until recently mined.  We performed a retrospective analysis of  310 cases of patients with breast cancer, who were exposed for at least ten years to different levels of ionizing radiation around their homes, to verify whether a correlation existed between disease incidence, prevalence, and exposure. Materials and Methods: Gamma radiation was measured on the roads and the urban street grid.  We retrieved the clinical files of 310 patients from the Population-Based Cancer Registry of Poços de Caldas city, MG, Brazil and compared the local prevalence and incidence of breast cancer per city district to the local effective doses.  Results: Effective doses of radiation around patients’ homes varied from 0.72 and 1.30 mSv/year, with 70% of the homes exposed to doses > 1.0 mSv/year. When considered the number of cases in the study in relation to the adult female population of the city, the incidence of female breast cancer was 25.9% higher than the national average incidence for the same period, 2003-2011 (68.32/100,000 versus 50.61/100,000 respectively). Conclusion: The higher incidence of breast cancer among the adult female population of Poços de Caldas may be associated with chronic exposure for ten or more years to effective doses equal or slightly above the international reference dose of 1.0 mSv/year. Other known risk factors for breast cancer in our patients were not different from those found nationwide.