While osteoporosis is a major public health concern in the developed world, little research regarding factors influencing bone mineral accrual in children has been conducted in developing countries. South Africa is of particular interest since the incidence of hip fractures in South African Blacks has been reported to be amongst the lowest in the world (32; 253). In this thesis, the association of lifestyle factors; in particular physical activity (PA), socio economic status (SES) and dietary calcium intakes on the growing skeleton of Black and White South African children is investigated. After using accelerometry to validate a physical activity questionnaire (PAQ), in a convenience sample of South African Black, White, male and female children (n=30), fitness levels were assessed in a larger group (n=69) of similarly aged children, stratified by race and gender. Fittest subjects had significantly greater physical activity scores (p=0.022) as reported on the PAQ, lower body mass index’s (BMI) (p=0.001) and least percentage body fat (p=0.001) (as assessed using Dual Energy X-ray Absorptiometry (DXA), than least fit subjects. White males who reported to be significantly more active than all other groups on the PAQ were significantly fitter (p<0.001) than White females and Black males and females. The next study sought to determine whether differences observed in physical activity levels between groups showed an association with bone mineral content (BMC), density (BMD) and area (BA) (as assessed using DXA). PA was analyzed in terms of a metabolic (METPA; weighted metabolic score of intensity, frequency, and duration) and a mechanical (MECHPA;sum of all ground reaction forces multiplied by duration) component for 386 children aged 9.5 (0.04) years recruited from a longitudinal birth cohort study. White children expended a significantly greater energy score (METPA of 21.7 (2.9)) than Black children (METPA of 9.5 (0.5), p< 0.001). When children were divided into quartiles according to the amount and intensity of sport played, the most active White children had significantly higher (p<0.05) whole body BMD and higher hip and spine BMC and BMD after adjustment for body size than less active children. White children in the highest MECHPA quartile also showed significantly higher (p<0.05) whole body, hip, and spine BMC and BMD after adjustment for body size than those children in the lowest quartile. No association between PA and bone mass of Black children was found. No significant differences between METPA and MECHPA quartiles and BA were observed for any group. Given the disparate backgrounds from which many South African children come, the next study sought to determine whether differences in socio-economic status between Black and White South African children influence PA patterns. This study explored the relationship between socio-economic status, PA anthropometric and body composition (via DXA) variables in 381 children aged (9.5 (0.04) years) recruited from a longitudinal birth cohort study . Children falling into the highest socio-economic status quartile had mothers with the highest educational levels, generally came from dual parent homes, were most physically active, watched less television, weighed more and had greater lean tissue than children in lower socio-economic quartiles (p<0.001). Significantly greater levels of lean mass (p<0.001) with increased activity level were observed after controlling for television watching time and fat mass. There were high levels of low physical activity and high television watching time among lower socio-economic status groups. White children were found to be more active than Black children, more likely to be offered physical education and to participate in physical education classes at school and watched less television than Black children. The final study sought to investigate the association between habitual PA patterns and dietary calcium intakes with bone mass acquisition over a one year period in 321 pre-pubertal South African children recruited from a longitudinal birth cohort study. Data were analyzed by regressing change in BMC and BA from age nine to ten years, against BA (for BMC), height and body weight. The residuals were saved and called residualized BMCGAIN and BAGAIN. Residualized values provide a good indication of weight, height and BA-matched accumulation rates. White children had significantly higher PA levels and calcium intakes than Black children. Most active White males had significantly higher residualized BMCGAIN and BAGAIN at the whole body, hip and spine but not at the radius, than those who were less active. Most active White females had significantly higher residualized BAGAIN at all sites except the radius than less-active girls. No such effects were seen in Black children. There was no interactive effect on residualized BMCGAIN or BAGAIN for calcium intake and PA in boys or Black girls, but an interactive and possible synergistic effect of calcium and physical activity was observed at the spine, radius and hip in White girls. In this population, PA has an osteogenic association with White children, but not Black children, which may be explained by the lower levels of PA in the Black children. Despite this, Black children had significantly greater bone mass at the hip and spine (girls only) (p< 0.001) even after adjustment for body size.In conclusion, differences between White and Black children’s PA levels were observed, with White children reporting higher PA levels and exhibiting higher fitness levels than Black children. Physical fitness correlated well with self reported physical activity levels on the PAQ and objectively measured body composition. Socio-economic status differences between White and Black children are highly related to differences in physical activity patterns and body composition profiles. Bone mass and area gain is accentuated in pre- and early-pubertal children with highest levels of habitual physical activity. Limited evidence of an effect of dietary calcium intakes on bone mass in boys and Black girls was found. The role of exercise in increasing bone mass may become increasingly critical as a protective mechanism against osteoporosis in both South African race groups, especially because the genetic benefit exhibited by Black children to higher bone mass may be weakened with time, as environmental influences become stronger.