The attainment of optimal peak bone mass during adolescence is important in the primary prevention of osteoporosis. Exercise may contribute to skeletal development and bone density during growth, although competitive exercise is suggested to have an adverse effect. This study assesses the effect of moderate exercise on the bone density of adolescent females. Additionally, other factors which significantly influence attainment of peak bone mass were identified. This was a cross-sectional study of 42 adolescent females, classified as runners (n=15) or non-runners (n=27). Nutrient intake, energy expenditure, menstrual history and pubertal stage were recorded. Bone age, skinfold thickness, body composition and bone mineral density (BMD) of total body, lumbar spine and proximal femur were measured. Statistical analyses used Student's t-test, Pearson correlation and multiple regression analyses. Runners had lower fat mass and higher lean mass, with a trend to higher BMD in all sites measured. There were no significant differences in menstrual cycle regularity, age at menarche or number who had attained menarche. Pubic hair development was similar in both groups. Breast development was delayed in runners, although this may have been a function of lower fat mass in this group. When subjects were categorized according to menarchal status, postmenarchal girls were significantly taller and heavier, with higher fat mass and significantly higher total body and lumbar spine BMD. There was no significant relationship between BMD at any site and dietary nutrient intake. Multiple regression analyses, using BMD as the dependent variable, identified running status, pubertal stage, fat mass and lean mass as significant determinants. When BMD/height was used, significant determinants in total body BMD were fat mass, pubertal stage and running status, while in the lumbar spine, only the latter two variables remained significant. In conclusion, body composition, physical activity and sexual maturity were identified as significant determinants of bone density during adolescence. Runners had significantly lower body fat than non-runners, but this did not interfere with hormonal cyclicity, and caused no detriment to their bone density. The results of this study are reassuring, since they indicate that sub-elite competitive athletics has no detrimental effect on bone mass accrual in adolescent females.