Skeletal size and mass are genetically programmed. Optimum skeletal size can be attained if the nutrient supply, ie, calcium, is ample, but the age-dependent decrease in skeletal mass that begins in the third decade cannot be arrested by adequate calcium intake alone. The decrease in skeletal mass is primarily caused by the age-dependent decrease in gonadal hormones. The dramatic drop in hormones in menopause is associated with a sharp decrease in trabecular bone and a slower decrease in cortical bone. In men this decrease is gradual. Replacement therapy with gonadal hormones can markedly slow this decrease in bone mass, provided calcium intake is adequate. Soluble forms of calcium are preferred to ensure adequate calcium absorption. Vitamin D supplementation beyond the recommended dietary allowance does not appear beneficial in osteoporosis, but may be so in cases of senile hyperparathyroidism. Calculations based on bone calcium turnover indicate that the recommended dietary allowance for calcium is adequate for boys and men, but is insufficient for adolescent girls. Calcium intake by women is probably too low to slow bone calcium turnover to its programmed minimum. Adequate calcium intake in childhood and adolescence is essential to attain the optimal bone mass and size.