Bone mass has been proposed as a marker of cumulative exposure to oestrogen in women. We have studied the association between bone mass and breast cancer in postmenopausal women. In 126 cases of breast cancers and 126 controls, the bone mineral density (BMD) of the lumbar spine (L2-L4), femoral neck, trochanter and Ward's triangle was measured by dual-energy X-ray absorptiometry. All cases of cancer were confirmed by pathological reports. A questionnaire including information on reproductive history and other variables was collected. BMD was significantly higher among breast cancer patients than controls at all sites, except at the femoral neck where BMD was increased in the cancer group, but not significantly. After adjustment for potential confounding factors, the estimated relative risk of breast cancer in the highest quartile of BMD compared to the lowest quartile ranged from 2.5 to 4.8 for various sites of measurement. These results confirm that bone-mass density is a strong predictor for breast cancer in postmenopausal women. Women in the lowest quartile of bone mass appear to be protected against breast cancer. The mechanisms underlying this relation may be explained by cumulative exposure to oestrogen.