This study tested the hypothesis that differences in mechanical loading regime was important when evaluating the potential role of physical activity on bone density in children. Seventeen children competing regularly in weight-bearing sports producing loads of at least 3 times body weight (Impact Load) were matched for race, gender, stage of puberty, body weight, and average daily training time with children involved in competitive swimming (Active Load). Bone mineral density (BMD) was measured using dual photon absorptiometry at the lumbar spine (L2-L4) and femoral neck (FN), Tanner staging was used to assess puberty, diet was evaluated based on 3-d dietary records from two occasions, and a questionnaire assessed average daily nonweight-bearing hours. There were no significant differences in age (13.2 ± 0.4 and 12.6 ± 0.4), height (154.9 ± 2.9 and 157.6 ± 3.0), or weight (43.6 ± 2.7 and 44.5 ± 2.2) between Impact and Active Load groups. Impact Load children had significantly greater FN BMD (0.78 ± 0.02) than Active Load children (0.72 ± 0.02) and a tendency for greater BMD L2-L4; 0.70 ± 0.03 and 0.66 ± 0.03, respectively. These data indicate that children involved in sports producing significant impact loading on the skeleton had greater femoral neck bone density and a trend for greater spinal bone density, than children in sports producing loads to bone primarily through muscular contraction.