Primary: To examine the relationship between relative timing of puberty with bone mineral density (BMD) in a group of adolescent girls; Secondary: To determine if family history of breast cancer was associated with bone mineral density.

Longitudinal study of girls recruited between 6 and 7 years of age seen every 6 months for 5 years, and subsequently seen annually. BMD of the lumbar spine was measured by dual-energy X-ray absorptiometry (DXA) at mean age of 12.5 years; age- and race-specific Z-scores (BMDz) were calculated. Age of pubertal onset was determined by the first occurrence of breast stage 2, and participants were categorized into race-specific early, on-time and late puberty onset groups.

BMDz by timing of pubertal onset, and by family history of breast cancer.

DXA scans were performed on 227 study participants, and a second scan was performed on 114 participants 2 years later. Age of onset of puberty was inversely correlated with BMDz, r = −0.31 (P < .0001). There was no association between BMDz and family history of breast cancer.

Earlier timing of puberty was associated with higher BMD. The high shared variance of BMD and timing of pubertal onset implies an underlying biologic basis.