A patient with primary shunt hyperbilirubinemia associated with Gilbert's syndrome is described. The laboratory findings of unconjugated hyperbilirubinemia, mild reticulocytosis, normoblastic erythroid hyperplasia of bone marrow, increased plasma iron turnover, and normal peripheral red blood cell survival were consistent with increased intramedullary hemolysis. Nevertheless, unconjugated bilirubin level exceeding 4 mg/100 mL, in the absence of appreciable anemia and of biochemical evidence of hepatic dysfunction, suggested the coexistence of defective hepatic bilirubin clearance. The mode of inheritance and the effect of fasting and phenobarbital on bilirubin concentrations were indicative of associated Gilbert's syndrome.