UNLABELLED: Endurance runners with low bone mass during adolescence may risk attaining a low peak bone mineral density (BMD) in adulthood. Alternatively, they may mature late and undergo delayed bone mineral accumulation. PURPOSE: The purpose of this study was to evaluate 40 adolescent runners (aged 15.9 ± 0.2 yr) at two time points, approximately 3 yr apart, to assess bone mass status and identify variables associated with bone mass change. METHODS: Follow-up measures included a questionnaire to assess menstrual status, training, and sports participation history, height and weight, and a dual-energy x-ray absorptiometry scan to assess total body, total hip, and lumbar spine BMD, bone mineral content (BMC), BMD z-score, and body composition. We used -1 and -2 BMD z-score cutoffs to categorize runners with low bone mass. RESULTS: Eighty-seven percent of girls with low BMD at baseline had low BMD at the follow-up. Girls with low compared with normal baseline BMD had lower follow-up adjusted total body (2220.4 ± 65.8 vs 2793.1 ± 68.2 g, P < 0.001), total hip (27.0 ± 1 vs 33.9 ± 1.0 g, P < 0.05), and lumbar spine (47.8 ± 2.0 vs 66.3 ± 2.2 g, P < 0.001) BMC values. Variables related to 3-yr training volume, menstrual function, age, developmental stage, and change in body mass explained 29%-54% of the variability in BMC change. CONCLUSIONS: The majority of adolescent runners with low BMD at baseline had low BMD after a 3-yr follow-up. Our observations suggest that "catch-up" accrual may be difficult and, thus, emphasize the importance of gaining adequate bone mineral during the early adolescent years.