The intestinal absorption of riboflavin, a vitamin which is absorbed by a specialized transport process in the proximal small intestine, was studied in nine infants and children with biliary obstruction (mainly due to congenital biliary atresia). After oral administration of a saturation dose of the vitamin (150 mg/sq m), the absorption of riboflavin, as shown by urinary recovery and maximum excretion rate data, was significantly impaired in the patients as compared to normal children of similar ages. Surgical correction of biliary atresia in two patients led to increased absorption of the vitamin.