The purpose of this study was to investigate the effects of physical training and calcium intake on bone mineral density (BMD) of students with mental retardation. Forty mentally retarded boys (age 7-10 years old) were randomly assigned to four groups (no differences in age, BMD, calcium intake and physical activity): training groups with or without calcium supplementation (Tr+Ca+ and Tr+Ca-) and nontraining groups with or without calcium supplementation (Tr-Ca+ and Tr-Ca-). The intervention involved 45 min of physical training performed 3 sessions a week and/or the addition of dietary calcium-rich food using enriched cow milk with vitamin D containing 230 mg calcium per serving, over 6 months. Paired-samples t-test and ANOVA analysis was used to determine the main and combined effects of training and calcium on BMD. All groups showed greater femoral neck BMD after 6 months. The increase in femoral neck BMD in the Tr+Ca+ group was 10% greater than increase in the Tr+Ca- group (not significant). Apparently, the effect of training was greater than calcium intake because the Tr+Ca- group achieved 4% greater BMD than Tr-Ca+ group (not significant). In this study, both training groups had greater BMD than the control group (Tr-Ca-) (P less than 0.05). In these participants with inadequate calcium intakes, additional exercise and calcium supplementation resulted in a 6-20% greater increase in BMD than controls at the loaded site (femoral neck). These results help to provide more evidence for public health organizations to deal with both exercise and nutrition issues in children for the achievement of peak BMD. (Contains 4 tables and 1 figure.)