Viral vectors based on adeno-associated virus (AAV) are showing exciting promise in gene therapy trials targeting the adult liver. A major challenge in extending this promise to the pediatric liver is the loss of episomal vector genomes that accompanies hepatocellular proliferation during liver growth. Hence maintenance of sufficient transgene expression will be critical for success in infants and children. We therefore set out to explore the therapeutic efficacy and durability of liver-targeted gene transfer in the challenging context of a neonatal lethal urea cycle defect, using the argininosuccinate synthetase deficient mouse. Lethal neonatal hyperammonemia was prevented by prenatal and early postnatal vector delivery; however, hyperammonemia subsequently recurred limiting survival to no more than 33 days despite vector readministration. Antivector antibodies acquired in milk from vector-exposed dams were subsequently shown to be blocking vector readministration, and were avoided by crossfostering vector-treated pups to vector-naive dams. In the absence of passively acquired antivector antibodies, vector redelivery proved efficacious with mice surviving to adulthood without recurrence of significant hyperammonemia. These data demonstrate the potential of AAV vectors in the developing liver, showing that vector readministration can be used to counter growth-associated loss of transgene expression provided the challenge of antivector humoral immunity is addressed.