Long-term corticosteroid therapy is the most frequent and most severe cause of iatrogenic osteoporosis. Hypocalcaemia, subsequent to the induced negative calcium balance, may lead to secondary hyperparathyroidism. Corticosteroids also affect bone itself, probably by disrupting the production of growth factors. Bone resorption increases and bone formation decreases leading to a reduction in total bone mass. The relative immobilization resulting from the corticoid-induced myopathy or the underlying disease may accelerate the process. On the average, after one year of treatment, 5% of the bone mass is lost, and loss may reach as much as 10 to 30% at certain sites. Nearly 40% of all subjects on long-term corticosteroids suffer fractures. Other iatrogenic causes include anticonvulants which perturb phosphocalcium metabolism, 1-thyroxin which leads to bone loss when administered for hormone substitution, gonadotropin-releasing hormone antagonists which inhibit the hypophyseal-ovarian axis, tamoxifen (used in the treatment of breast cancer) which has an oestrogen-like effect, and other circumstances such as chemotherapy and long-term heparin. The gravity of iatrogenic osteroporosis thus requires preventive measures. Calcium and vitamin D supplements can compensate for impaired intestinal absorption of calcium but have no effect on bone density. One-alpha hydroxyl derivatives have been suggested but their effect remains controversial. Calcitriol can prevent bone loss in the lumbar vertebrae but hypercalcaemia occurs in one-fourth of the cases, limiting its use. Recent reports have shown that anti-oestroclastic agents may be useful. Nandrolone decaonate would have a favourable effect on bone loss but also causes virilization. In patent osteoporosis, fluorine can be combined with calcium resulting in increased lumbar bone density. Calcitonin and calcium can also be combined to induce a rise in bone density. The long-term effects of these treatments in terms of reduced fracture risk remain to be determined. A better understanding of the adverse effects of the different classes of corticosteroids is essential for optimal treatment. In cases requiring long-term therapy implicating the risk of iatrogenic osteoporosis, bone density quantitation can be a valuable means of evaluating bone loss, and of adapting preventive or corrective measures.