Serum, red cell, urinary and bone Mg levels have been measured in 10 post-menopausal osteoporotics. Osteoporosis was confirmed by clinical fracture, histological analysis and dual-photon absorptiometry quantification. When compared to a healthy population matched for age and sex, serum Mg levels were statistically significantly (p less than 0.05) lower in osteoporotics (0.82 +/- 0.06 mmol/l) compared to normal subjects (0.87 +/- 0.04 mmol/l). No difference appeared for red cell, urinary or bone Mg determination. If these results are confirmed in a long-term study, serum Mg might be added to a batch of tests used to discriminate a subset of patients with an increased risk of developing postmenopausal osteoporosis.