Imagine that your patient needs a transplant, say a new heart. A specially bred pig carrying human genes may soon supply it. That is the hope and hype of xenotransplantation, the transfer of animal cells, tissues, and organs to humans. Xenotransplantation is not entirely novel, as pig heart valves have been used for many years without apparent ill effect, but they are essentially inert tissue and seldom elicit rejection. There is now considerable excitement that the transplantation of live animal tissues may soon become a practical treatment option, although this is matched by concern over the risk of new zoonotic infections in transplant recipientsh.1 This article discusses the potential and the problems of xenotransplantation and explains why it faces an uncertain future.