The pig is the favorized donor species for clinical xenotransplantation. However, PATIENCE et al. could show, that porcine endogenous retroviruses (PERV), released by a porcine kidney cell line, are capable of infecting human cell lines in vitro. Based on this discovery there is an ongoing discussion concerning the risks of zoonosis combined with xenotranplantation, which culminated in the demand for a moratorium on clinical transplantation of porcine organs. Recent findings exclude the possibility of an artifact due to the use of an immortalized cell line: Release of infectious PERV was also shown for mitogenic stimulated primary porcine peripheral blood mononuclear cells and, even more important, for primary porcine endothelial cells. In contrast, none of the recent retrospective in vivo studies showed evidence for PERV transmission, neither in patients after transplantation of porcine pancreas islet cells or after extracorporal perfusion of porcine kidneys, nor in baboons after transplantation of porcine endothelial cells. Currently it is not known, whether impairments of the immunological responses against foreign pathogens, which are associated with different xenotransplantation strategies, could enable PERV in vivo infection. Only in vivo experiments, if possible in suitable subhuman primate models, offer the prospect for a final risk assessment.