Tamoxifen, which is used for treating breast cancer, exhibits estrogenic and antiestrogenic characteristics, depending on the tissue. In the human breast it acts as an antiestrogen, whereas estrogenic effects have been reported on endometrium and bone. The purpose of this study was to determine whether tamoxifen (TAM) prevents bone loss in elderly, postmenopausal women. Bone mineral density of the lumbar spine (SBD) was measured in elderly women (at least 10 years after menopause) 5 years after stage I or II breast cancer (n = 111). The results showed that SBD in untreated patients (n = 74) was significantly lower (p < 0.05) than SBD in patients (n = 37) treated with TAM over 5 years. In a subgroup of patients (n = 24) with positive estrogen receptor status, changes in SBD 12 months after discontinuation of 5-year TAM therapy were measured and compared with the changes of extended TAM treatment over a sixth year. Twelve months after withdrawal of 5-year TAM medication (n = 11) bone density decreased significantly (- 4.8+/-2.5%; p > 0.05), whereas in the group of women (n = 13) receiving extended TAM medication (20 mg) for an additional 12 months, SBD ( + 1.9+/-3.5 %) was maintained during the observation period, and was significantly higher when compared with the group of untreated patients (p <0.05). We conclude that tamoxifen has a preventive effect on trabecular bone loss at the lumbar spine, when compared to age-matched data and to untreated women with breast cancer in the late menopause. Our data give evidence of benefits to bone density provided by prolonged administration in patients after breast cancer and at risk of osteoporosis.