Purpose: Bone mass has been proposed as a marker of cumulative exposure to estrogen in women. We have studied the association between bone mass and the risk of breast cancer in postmenopausal women. Methods: We investigated the association between bone mineral density (BMD), as measured at the lumbar spine and femoral neck, and the risk of breast cancer in women age 50 or older who had received an initial diagnosis of stage 0III breast cancer confirmed by pathologic assessment of breast tissue. We recruited 218 women with newly diagnosed breast cancer at Asan Medical Center from January 1, 2003 to December 31, 2004 and 963 women whose BMD was measured at the Health Promotion Center of Asan Medical Center as controls. Groups were divided by age: 5059 years old (Group A), 6069 years old (Group B), and over 70 years old (Group C). We compared BMD and prevalence of osteoporosis between groups. BMD was measured by lunar EXPERT-XL for breast cancer patients and Hologic QDR 4500-A for control group. A cross-calibration equation was used to compare BMD by different dual X-ray absorptiometry systems. Results: BMD was significantly higher among breast cancer patients than controls at lumbar spine (p=0.04); femur neck BMD was higher but not statistically significant (p=0.47). After adjustment for age, the estimated odds ratio was 4.46 (p=0.02). In Group A, BMD for spine and femur neck was significantly higher in breast cancer patients (p<0.05). In Group B,spine BMD spine was significantly higher in breast cancer patients (p=0.005); femur neck BMD was higher but not statistically significant (p=0.24). In Group C, BMD for spine and femur neck was higher in breast cancer patients but not statistically significant (p>0.05). Prevalence of osteoporosis of the spine and femur neck was 14.9 and 4.6% for breast cancer patients and 19.6 and 8.3% for controls. Conclusion: These results show that high bone mineral density has a strong relationship among breast cancer patients in postmenopausal women. In patients 70 years old or over, the age effect for bone mineral density decreased the effect of estrogen on bone.