The occurrence of anticonvulsant osteomalacia was studied in 31 epileptic inpatients, 16 women and 15 men. Disturbances in biochemical parameters indicating osteomalacia were frequent. Thirty two per cent of the patients were hypocalcemic, 55% had an increase in S-ALP and 26% in U-HOP, and dU-Ca was decrease in 55%. The S-25OHD3 concentrations were significantly lower in the patients compared with healthy controls. BMD was decreased in females but not in males compared wih the controls. Histomorphometric analysis revealed an increase in the amount of osteoid, but the amount of trabecular bone was no lower than in the controls. The amount of resorption surfaces was increased in the females, but not in the males. The patients who took less physical activity had a pronounced decrease in BMD. The conclusion drawn was that osteomalacia is a frequent complication of long-term anticonvulsant medication, especially among institutionalized patients.