Dietary changes resulting from the post-World War II occupation of Okinawa by the US military have been largely deleterious, resulting in a marked increase of obesity among Okinawan residents. In this study, we examined the association between BMI and the risk of developing breast cancer according to the menstruation status and age, and the correlation between BMI and expression of estrogen receptor (ER). Breast cancer cases were 3,431 females without any personal or family history of breast cancer. Control subjects were 5,575 women drawn from the clinical files of Nahanishi Clinic. We found that women, who were overweight or obese, regardless of menopausal stage, had a higher risk of breast cancer compared to women with normal weight and this difference was statistically significant (p < 0.001, respectively). This risk was especially apparent in older (> 40 years) overweight or obese women. The women who were overweight or obese during postmenopausal ages were at higher risk of ER-positive breast cancer compared to women with normal weight. Results of our present study clearly indicate that increased BMI was associated with increased risk of developing breast cancer in Okinawan women, regardless of menopausal status. In addition, there was statistically significant correlation between BMI and ER expression in the postmenopausal period. Given the obesity epidemic associated with the extreme sociological and dietary changes brought about by the post-war occupation of Okinawa, the present study provides essential guidelines on the management, treatment and future breast cancer risk in Okinawa.