Background: The phase III DATA study (NCT00301457) investigates the efficacy of 6 versus 3 years of adjuvant anastrozole (AI) after an initial 2-3 year treatment with tamoxifen in postmenopausal women with breast cancer. In a planned side study, we assessed patterns of care in detection and treatment of osteopenia/osteoporosis and linear trends over time in bone mineral density (BMD). Methods: BMD measurements and bisphosphonate use in the DATA study were left to the treating physician. We registered all BMD measurements and start of bisphosphonate-use. BMD was measured by a dual-energy x-ray absorptiometry (DEXA) scan of the lumbar spine and/or the hip and transformed to a T-score. Time to osteopenia or osteoporosis was analysed by reverse Kaplan Meier methodology. For the linear trend in BMD T-score, linear mixed models with random effects for patient were used. Results: Of the 1860 eligible patients in the DATA study, 910 (48.9%) had a BMD measurement at the start of anastrozole (baseline). In 417 (45.8%) patients the baseline measurement indicated a normal BMD, in 408 (44.8%) osteopenia, and in 85 (9.3%) osteoporosis. In patients with a normal baseline BMD, osteopenia was observed in 53.5% of patients in the 6-year group, and 55.4 % in the 3-year group (p = 0.18), while none developed osteoporosis within 6 years after randomization. Of the patients with osteopenia at baseline, 24.4 % developed osteoporosis in the 6-year group and 20.4% in the 3-year group (p = 0.89). The yearly BMD change in the lumbar spine showed a decline of the mean T score of 0.075. When bisphosphonates were added the decline became 0.047 per year (p < 0.001), and when the AI was stopped and bisphosphonates continued the BMD increased yearly with 0.047. Conclusions: In postmenopausal women who had been treated with 2-3 years of adjuvant tamoxifen, BMD measurement showed osteopenia in 44.8% of women and osteoporosis in 9.3%. Although subsequent AI-use was associated with a further decrease of BMD, none of the patients with a normal BMD at baseline developed osteoporosis regardless of treatment arm. Clinical trial information: NCT00301457