Larger and more perfect crystals in bone mineral from osteoporotic women have been described using biophysical methods. In search of the cause of this change, both chemical analysis and infrared spectrophotometry were used on iliac crest bone biopsies from 19 postmenopausal osteoporotic women. These women had each had a vertebral crush fracture and had a urinary hydroxyproline: creatinine ratio greater than 0.012. Lower than normal trabecular bone magnesium content and larger and more perfect crystals in trabecular bone mineral identified on infrared spectrophotometry were found in 16 out of the 19 women. Magnesium deficiency was confirmed by Thoren's magnesium load test in this subgroup of 16. Higher than normal bone magnesium content and smaller and less perfect crystals in bone mineral were found in five postmenopausal uremic women tested.