Calcium and vitamin D metabolism were evaluated in 5 adult epileptic patients before and during treatment with phenytoin. Significant decreases occurred in serum concentrations of calcium, albumin, and 25-hydroxy-cholecalciferol. The decreases in serum calcium paralleled those in serum albumin. Significant increases occurred in serum alkaline phosphatase and 1 alpha, 25-dihydroxycholecalciferol, in urinary hydroxyproline, and in the fractional gastrointestinal absorption of calcium. Urinary cyclic adenosine monophosphate and serum parathyroid hormone did not change. The results suggest that the bone disease resulting from phenytoin therapy may be associated with a deficiency of 25-hydroxycholecalciferol and not of 1 alpha, 25-dihydroxycholecalciferol, and that reduced gastrointestinal absorption of calcium or changes in parathyroid function may not be necessary for the development of bone disease.