The adaptation of bone to exercise has been shown to be modified by dietary calcium intake. The aim of this randomised controlled trial was to investigate whether there was a differential response to calcium supplementation in elite gymnasts and school children controls. The primary hypothesis was that gymnasts who took calcium supplements would have greater increases in cortical and trabecular volumetric bone mineral density (vBMD) at the radius and tibia. Secondary outcomes studied were changes in bone geometry at the radius and tibia and lumbar spine and whole body measurements. Children were randomised to 12 months daily supplementation of 500 mg elemental calcium (1250 mg (in the form of calcium carbonate salt)) or placebo. Outcome measures were assessed using peripheral quantitative computed tomography (pQCT) (distal and diaphyseal radius and tibia) and dual energy X-ray absorptiometry (DXA) (lumbar spine and whole body). Eighty-six subjects participated in the trial (44 gymnasts, 42 controls) and 75 subjects completed the trial (39 gymnasts, 36 controls). Data were analysed by analysis of covariance adjusting for baseline value of bone parameters, age, height, gender and puberty, and delay between baseline measurement and start of intervention. The primary analysis was for a calcium-exercise interaction; a pooled calcium effect with no interaction was also tested. Results are presented as ratios (95% confidence intervals). At the distal tibia, trabecular vBMD showed a significant interaction (p=0.04), with controls (1.00: 0.99, 1.09) responding more than gymnasts (0.98: 0.94, 1.02) to supplementation. At the distal radius, change in trabecular vBMD was not significant (p=0.05). There were no differences in change in cortical vBMD at either site between the gymnasts and controls (tibia: p=0.82, radius: p=0.88). For all other secondary outcomes at radius, tibia, spine and whole body no significant interactions were found. In conclusion, there was no beneficial effect of additional calcium in gymnasts who already consume their recommended nutrient intake (888 mg/day; United Kingdom reference nutrient intake for 8- to 11-year-olds is 555-800 mg/day) for calcium. We speculate that gymnasts have already adapted their bones (geometry and vBMD) to the demands imposed upon them by the loading they are subjected to during gymnastics and do not benefit from additional calcium supplementation.