We report on the deliberations of an interdisciplinary group of experts in science, law, and philosophy who convened to discuss novel ethical and policy challenges in stem cell research. In this report we discuss the ethical and policy implications of safety concerns in the transition from basic laboratory research to clinical applications of cell-based therapies derived from stem cells. Although many features of this transition from lab to clinic are common to other therapies, three aspects of stem cell biology pose unique challenges. First, tension regarding the use of human embryos may complicate the scientific development of safe and effective cell lines. Second, because human stem cells were not developed in the laboratory until 1998, few safety questions relating to human applications have been addressed in animal research. Third, preclinical and clinical testing of biologic agents, particularly those as inherently complex as mammalian cells, present formidable challenges, such as the need to develop suitable standardized assays and the difficulty of selecting appropriate patient populations for early phase trials. We recommend that scientists, policy makers, and the public discuss these issues responsibly, and further, that a national advisory committee to oversee human trials of cell therapies be established.