OBJECTIVE: To evaluate bone mineral density (BMD) by age in premenopausal elite long-distance runners at 2 time points, 5.02 ± 0.5 years apart. DESIGN: Follow-up study. SETTING: University Research Institute. PARTICIPANTS: Twenty-three elite-level distance runners (baseline age, 24.9 ± 3.9 years; body mass index, 18.7 ± 1.1; running distance, 89.3 ± 19.4 km/wk) participated in the study. MAIN OUTCOME MEASURES: Dual-energy x-ray absorptiometry (iDXA; GE Healthcare)-derived BMD z-scores for the total body (TB), lumbar spine (LS) (L2-L4), and proximal femur. A z-score of <-1.0 indicated low BMD by age. RESULTS: Lumbar spine BMD z-scores had increased significantly by follow-up for all runners (baseline, -1.1 ± 0.9; follow-up, -0.7 ± 0.7; P = 0.04) and for formerly amenorrheic runners with restored menstruation (n = 10; baseline, -1.5 ± 0.9; follow-up, -0.9 ± 0.6; P = 0.03). Total body, but not proximal femur z-scores, had also increased (P < 0.05). Improvements in z-scores correlated with an increased body fat (LS: R2 = 0.27; TB: R2 = 0.35; P < 0.01) and menstrual history score (LS: R2 = 0.36; TB: R2 = 0.27; P < 0.05). There were no associations with lean mass or running volume at any skeletal site (P = 0.07-0.89). CONCLUSIONS: Low BMD by age in highly-trained female runners may be at least partially reversible before the age of 30 years, even when competitive running is continued. This seems to be related to restored menstruation and an increased body fat. Further larger studies are required to clarify our findings.