Organ failure is treated either with an artificial organ or by organ transplantation. However, both approaches have fundamental problems such as medical cost, ethical issues, and absolute shortage of organ donors. As an alternative, regenerative medicine utilizing stem cells has been a focus of attention. Current stem cell therapy, however, mainly targets diseases that can be treated by cell transplantation. This is because organogenesis requires complex interactions among cells and organs during development, making regeneration of a solid organ effectively impractical to date. Our goal is to generate donor ES or iPS cell-derived solid organs in xenogenic animals using blastocyst complementation to help patients with organ failure and to provide human materials for drug discovery and health-care related industries.