Experience from 73 consecutive patients with non-hemolytic, isolated hyperbilirubinemia indicates that such patients almost without exception have increased serum levels of unconjugated bilirubin (greater than 17 mumol/l) and a conjugated bilirubin of less than 30% of total. Normal total bilirubin values were observed in 15% of 500 determinations, although some subjects never had normal values. The reduced caloric intake test had a low 'sensitivity' as a test for diagnosing Gilbert's syndrome, particularly in females. Long-term (9-29 years) follow-up study demonstrated that hyperbilirubinemia is lifelong and not associated with increased morbidity or deficiency of coagulation factors II, VII, and X. The data suggest that no further medical study is necessary in subjects with moderate isolated hyperbilirubinemia and normal blood reticulocyte count.