Alterations in vitamin D metabolism are generally thought to account for the hypocalcemia and osteopenia caused by long term treatment with anticonvulsant drugs. Regional variation in the incidence and severity of anticonvulsant drug-induced bone disease has been attributed to differences in sunlight exposure, with most reports coming from areas with limited sunshine or from institutionalized patients. Serum ionized calcium levels in 109 ambulatory adult epileptic outpatients receiving chronic anticonvulsant drug therapy in Georgia were decreased [4.73 +/- 0.02 (+/-SE) vs. 4.97 +/- 0.01 mg/dl; P less than 0.001). Immunoreactive PTH concentrations were increased (5.5 +/- 0.4 vs. 4.0 +/- 0.3 microliterEq /ml; P less than 0.005), while bone mineral content was reduced, averaging only 88.8% of the predicted normal values. Hypocalcemia and osteopenia occurred in spite of normal mean levels of serum 25-hydroxyvitamin D and 1,25-dihydroxy-vitamin D. The indirect relationship between serum concentrations of antiepileptic drugs and the serum ionized calcium level, and the lack of correlation with vitamin D metabolite levels suggested that hypocalcemia was independent of the effect of the drugs on vitamin D metabolism. Bone biopsies revealed increased osteoid but normal calcification front formation, accelerated mineralization rate, and decreased mineralization lag time indicative of increased skeletal turnover, rather than osteomalacia.