Serum 25-hydroxy-vitamin D (25-OHD) concentrations were measured in 49 patients with hepatobiliary disease in infancy. Low mean values were found in groups of patients with biliary atresia, neonatal hepatitis, choledochal cyst, and chronic intrahepatic cholestatic syndrome. In the group of patients with surgically repaired biliary atresia, the mean value did not differ from normal. Parenteral vitamin D increased 25-OHD in serum in patients with biliary atresia, but did not do so in one patient with neonatal hepatitis. In contrast, oral vitamin D did not increase serum 25-OHD concentrations in patients with biliary atresia. It is concluded that the reduction of serum 25-OHD seen in biliary atresia was largely due to the malabsorption of vitamin D, while in neonatal hepatitis it was due to impairment of 25-hydroxylation of the vitamin.