This report details the first case of cardiac xenotransplantation in a neonate. The recipient, a victim of hypoplastic left heart syndrome (HLHS), survived 20 days. Autopsy findings are documented. The cardiac graft showed only traces of cell-mediated rejection. Graft failure appears to have resulted from a progressive, potentially avoidable humoral response, unmodified by immunosuppression. Cardiac allotransplantation and selective baboon-to-human xenotransplantation deserve further exploration as investigational therapy for neonatal HLHS.