To test the efficacy of a health plan-based lifestyle intervention to increase bone mineral density in adolescent girls. Two-year randomized, controlled trial. Large health maintenance organization. Girls 14 to 16 years old with body mass index below the national median. Intervention Behavioral intervention (bimonthly group meetings, quarterly coaching telephone calls, and weekly self-monitoring) designed to improve diet and increase physical activity. Total bone mineral density was measured by dual-energy x-ray absorptiometry. Behavioral outcomes included intake of calcium, vitamin D, soda, and fruits and vegetables; high-impact and strength-training physical activity; measures of strength and fitness; and biomarkers (osteocalcin and naltrexone). Compared with control subjects, girls in the intervention group had significantly higher bone mineral density in the spine and trochanter regions during the first study year, which was maintained during the second study year. The naltrexone biomarker demonstrated a greater relative decrease in the intervention group compared with the control group, with nonsignificant changes in osteocalcin consistent with more bone building in the intervention group. Participants in the intervention group reported significantly greater consumption of calcium in both study years, vitamin D in the first year, and fruits and vegetables in both years. We found no effect on soda consumption or target exercise rates. A comprehensive health care-based lifestyle intervention can effectively improve dietary intake and increase bone mineral gains in adolescent girls. To our knowledge, this study is the first to significantly improve bone mass in adolescent girls in a non-school-based intervention. Trial Registration ClinicalTrials.gov Identifier: NCT00067600.