We assessed bone mineral density (BMD) change with aromatase inhibitor (AI) treatment in a contemporary cohort of women with breast cancer treated in Kaiser Permanente Northern California. Percent and estimated annual percent changes in BMD at the total hip and lumbar spine were examined in 676 women receiving AI therapy who had two serial BMD reports available (at least 1 year apart) before and after AI initiation (N = 317) or during continued AI therapy (N = 359). BMD changes were examined at the total hip and lumbar spine and compared by age and clinical subgroups. Women experienced BMD declines after AI initiation or continued therapy, with median annual percent change – 1.2% (interquartile range, IQR – 2.4 to – 0.1%) at the hip and – 1.0% (IQR – 2.3 to 0.1%) at the spine after AI initiation, and – 1.1% (IQR – 2.4 to 0.1%) at the hip and – 0.9% (IQR – 2.4 to 0.5%) at the spine during continued therapy. Higher levels of bone loss were observed among younger (< 55 years) compared with older (≥ 75 years) women at the hip (- 1.6% vs. - 0.8%) and at the spine (- 1.5% vs. - 0.5%) after AI initiation, and at the hip (- 1.4% vs. - 1.2%) and at the spine (- 2.4% vs. - 0.001%) during continued therapy. Small but consistent declines in total hip and lumbar spine BMD were present in breast cancer patients following AI therapy initiation or continued AI therapy. Although the overall rates of osteoporosis were low, greater estimated levels of annual bone loss were evident among women < 55 years.