In 151 epileptic patients, who had been treated with phenytoin for from 3 months to 31 years, the three indices of anticonvulsant osteomalacia: Serum calcium, serum alkaline phosphatase, and bone mineral content were studied to find the relation between the pathological changes and the duration of phenytoin treatment. The results indicate that anticonvulsant osteomalacia sets in quite early after institution of phenytoin treatment, and is kept at a constant level thereafter. If epileptics should have prophylactic vitamin D treatment, the extra supply of vitamin D probably should he given from the beginning of the anticonvulsant therapy.