The ability to use animal organs, such as from the pig, for the purposes of transplantation in humans, would clearly overcome the major shortage of human organs that greatly restricts transplantation programmes worldwide. Recent experimental advances have raised the possibility of renewed attempts at organ xenotransplantation in humans within the near future. Previous clinical experience, dating back to 1906, is briefly reviewed. The problems that still require resolution include the immunological barrier, the risk of transferring infectious agents with the transplanted organ, and uncertainty as to whether the transplanted animal organ will function satisfactorily in the human environment. Ironically, the answers to some of these problems may only be provided when clinical xenotransplantation is undertaken.