Crigler-Najjar type 1 disease (CN1) is a rare inherited metabolic disease characterized by complete absence of hepatic UDP-glucuronosyl transferase (UGT1), resulting in high levels of unconjugated bilirubin. CN1 is an attractive candidate disease for gene therapy. Here we show that in vivo neonatal hepatocyte transduction using recombinant oncoretroviral vectors results in long-term and complete phenotype correction in Gunn rats, a model for CN1. Two-day-old newborn Gunn rats were injected via the temporal vein with 200 microL UGT1 or control beta-galactosidase retroviral vectors. In UGT1-injected animals, bilirubinemia was normal at 6 weeks (3 micromol/L) and remained in the normal range (i.e., <10 micromol/L) for more than 34 weeks. In contrast, in beta-galactosidase-injected animals as well as in noninjected controls, bilirubinemia remained at a high level (i.e., >100 micromol/L) during the whole experimental follow-up. Large amounts of bilirubin monoglucuronides and diglucuronides were present in the bile of treated animals. Finally, polymerase chain reaction and reverse transcription polymerase chain reaction analysis as well as Western blot confirmed the presence and expression of UGT1 almost exclusively in the liver. The estimated proportion of transduced hepatocytes was in the range of 5% to 10%. In conclusion, complete and permanent correction of hyperbilirubinemia in newborn Gunn rats using retroviral vectors can be obtained, paving the way for future gene therapy for CN1.