Drug-induced osteomalacia appears to be a relatively common disorder in patients receiving long-term anticonvulsant drug therapy. The severity of clinical manifestations in any given individual appears to be a function of the combined effects of a variety of factors including drug type and total drug dose, dietary vitamin D intake, sunlight exposure, and physical activity level. Aided by the recent development of sensitive techniques such as the serum 25-hydroxyvitamin D assay and the photon absorption methods for bone mass determination, one can now detect abnormalities in vitamin D and bone metabolism with much greater precision. As a consequence, the incidence of disordered mineral metabolism in patients receiving long-term anti-convulsant therapy can be determined with greater precision and therapeutic regimens instituted to prevent the associated morbidity.