The inheritance of Crigler-Najjar type II disease is still contested. Autosomal dominant transmission with incomplete penetrance and autosomal recessive transmission have been proposed. We had the opportunity to study the hepatic activity of bilirubin uridinediphosphate glucuronyltransferase in parents whose first child had been affected by Crigler-Najjar type II disease. The demonstration of reduced activity of glucuronidation in the liver of both parents suggests autosomal recessive inheritance. The second infant of this couple was affected by the same disease and was treated with success by phenobarbital.