Recently an unconjugated hyperbilirubinaemia, without any other abnormalities in liver function test, in 14.3% of HBV Japanese carriers has been noticed. Therefore, it would be possible to argue that the persistent infection of HBV in hepatocytes might play a role in an hypothetical metabolic derangement of bilirubin clearance. Twenty-five subjects in a group of 468 HBsAg+ patients (equal to 5.33%) presented an hyperbilirubinaemia. This percentage was not different from the 5.83% found in 3083 HBsAg- controls coming from the same institution. Therefore we could exclude that in our population the presence of HBV surface antigen itself would determine a statistically higher level of total bilirubin (TB) than in controls. The nicotinic acid (NA) loading test may reveal some bilirubin metabolic defects (i.e. Gilbert syndrome), even in subjects with normal basal values of TB. According to this background, we performed in 11 HBsAg+ males with basal TB higher than 17.1 mumol/l (1 mg%) (group A/1), 13 HBsAg+ males with basal TB lower or equal to 17.1 mumol/l (group A/2) and 14 HBsAg- normal controls matched for sex and age (group B) the NA test according to Röllinghoff et al. All the parameters calculated by the NA test resulted significantly different in the A/1 group compared with the B group, but not different from those found by several authors in the Gilbert's syndrome. On the contrary, no significant differences have been noticed between the latter group and the A/2 group.(ABSTRACT TRUNCATED AT 250 WORDS)