Calcium and folic acid absorption were studied in 28 adult male epileptics on chronic anticonvulsant therapy. In 16 patients on diphenylhydantoin alone, calcium absorption was abnormal in 9. In 12 patients on both diphenylhydantoin and phenobarbital, calcium absorption was abnormal in 3 patients. Folic acid (3H-PGA) absorption was normal in all but one patient, while serum folate (<6.4 ng/ml) was reduced in all patients. Hypocalcemia (<8.5 mg/100 ml) occurred in only 2 patients, while serum alkaline phosphatase was elevated in 7 patients. These findings support the proposal that rickets and osteomalacia reported in patients on chronic anticonvulsant therapy results from reduced calcium absorption. The effect of these drugs appears to be the acceleration of the metabolism of vitamin D and an increase in the excretion of polar metabolites. This may result in reduced levels of 25-hydroxycholecalciferol and 1, 25-dihydroxycholecalciferol which are necessary for normal absorption of calcium. Since calcium absorption may be impaired secondary to a relative vitamin D deficiency, a supplemental increase in vitamin D intake by patients on anticonvulsant drugs is recommended. and; 1975 by The Endocrine Society.