Consideration of the ethics of human gene therapy does not seem to raise questions that have never been asked before. However, particularly when gene therapy is extended to modification of the germ cells, several ethical issues take on an added importance or significance. These issues are: (i) possible moral limitations on tampering with "human nature"; (ii) the extent of our responsibility to future generations; (iii) the appropriate use of early human embryos in genetic research. Furthermore, standard norms in clinical and research ethics require careful application to trials of human gene therapy, even if only somatic rather than germ-line improvements are sought.