Brainstem auditory evoked potentials (BAEP) were studied in five fullterm newborns before and during an exchange transfusion (ET) for hyperbilirubinemia and 1, 24, 48 hours after, in order to evaluate the changes of BAEP following acute decreases in bilirubinemia. Hyperbilirubinemia was due to ABO incompatibility. The newborns were free of other risk factors known to alter BAEP. In comparison with our normal laboratory data, we observed significant differences in pre-ET recordings for latencies I and V (p less than 0.01), and for interpeak latency V-III (p less than 0.01). After ET, there was a tendency towards shortening of all mean latencies and interpeak latencies in correlation with the acute decrease in bilirubinemia, but post-ET BAEP alterations consisted predominantly of a wave V latency and of a wave V-I interval shortening (p less than 0.01). In four patients, during the 48 hours post-ET we observed modifications of the wave V-I interval related to variations of bilirubinemia. At 12 months, BAEP were normal. Even acceptable increase of bilirubinemia in otherwise normal newborns may have a deleterious influence on BAEP and this could have implications for the determination of a so-called threshold of bilirubin neurotoxicity. Our results suggest that bilirubin neurotoxicity is rapidly reversed by ET, but it seems important to verify BAEP in the follow-up of hyperbilirubinemic newborns.