Hepatic bilirubin uridine diphosphate glucuronosyltransferase activity was assayed in an infant with prolonged jaundice and congenital hypothyroidism before thyroid therapy. This activity was nil, suggesting a possible delayed maturation of the enzyme. Although further studies will be necessary to confirm this hypothesis, prolonged jaundice associated with congenital hypothyroidism may be due to a delayed maturation of the hepatic glucuronidation of bilirubin.