We have prospectively studied forearm trabecular and cortical bone mineral density gain (ΔBMD) in relation to nutrient intake, weight-bearing physical activity (WPA) and daylight exposure (DE) in 470 healthy boys and girls aged 8.2-16.5 years at the baseline. BMD was assessed using single photon absorptiometry (SPA). Cortical ΔBMD peaked at the age of 14.0 ± 0.3 and 16.0 ± 0.3 (SD) years in girls and boys, respectively. Girls had achieved adult premenopausal values of trabecular BMD by the age of 15 years and cortical BMD values by the age of 16.5 years. WPA, BMD, body height, height gain, weight, weight gain, dietary polyunsaturated fat and sodium were correlated with ΔBMD. WPA, predicting cortical as well as trabecular ΔBMD, had the greatest effect on trabecular ΔBMD in the presence of a high calcium intake in children below 11 years of age. In conclusion, our results indicate that physical activity and calcium intake should be encouraged at a prepubertal age in order to increase bone density.