The objective of this study was to evaluate bone mineralization (BMC) and bone mineral density (BMD) of the osseous tissue in girls training swimming an being in the period of reaching the peak bone mass, as compared to girls being at a similar age and non-practicing sport, taking into account dietary allowances for calcium and phosphorus and dietary ratios of these elements. Both the swimmers and their non-training colleagues were found to meet nutritional demands to the same extent and their diets did not differ in the intakes of energy nor nutrients (protein, calcium, phosphorus), which is incorrect in the case of the non-training girls. An alarmingly low intake of calcium at a, simultaneously, excessive intake of protein and phosphorus, as well as incorrect ratios between calcium and phosphorus and between calcium and protein observed especially in the case of the swimmers, might have an adverse effect on the mineralization of osseous tissue in the period of reaching peak bone mass.