Larger and more perfect bone mineral crystals and decreased bone magnesium concentration were found in postmenopausal osteoporosis, senile osteoporosis, alcoholic osteoporosis and osteoporosis associated with thalassaemia. The decreased bone magnesium concentration and the increased retention of magnesium in the magnesium load test suggest magnesium deficiency in post-menoposal osteoporosis, probably caused by magnesium malabsorption.