Xenotransplantation is currently being investigated as a possible solution to the world-wide shortage of human organs. Several major advances have unquestionably been accomplished in the last decade in this field. These include an elucidation of the immunological mechanisms underlying the rejection of pig organs transplanted into primates, the generation of novel lines of genetically-engineered pigs that are more "compatible" with man and a better understanding of the risk of zoonosis related to the transmission of porcine endogenous retroviruses to primates. Though very encouraging, the existing preclinical data do not entitle clinical trials with pig organs to begin as yet and, for the time being, the use of human organs remains the only viable option for patients with terminal organ failure.