The word “serious” appears in laws and policies regarding genetics services but is not defined. Genetics professionals would most likely be consulted if definitions are made. We surveyed all U.S. board-certified genetics services providers and all members of the European Society of Human Genetics (ESHG), Canadian College of Medical Geneticists (CCMG), and Ibero-American Society of Human Genetics (IASHG), using anonymous, mailed questionnaires. Respondents were asked to list three conditions they considered lethal, three that were serious but not lethal, and three that were not serious. Of 3,317 asked to respond, 1,481 (45%) returned questionnaires. Analysis was limited to responses of the 1,264 (85%) who saw patients. Respondents listed 537 conditions, with extensive overlap between categories; 46% of conditions listed as serious were also listed as not serious and 41% were listed as lethal. Respondents did not want professional societies, laws, or national ethics committees to define serious. They favored individual patients as decision makers, with the help of individual doctors. Their reasons were that genetic disorders vary in expression, individuals perceive disorders differently, professional or legal definitions could be unfair to minority groups, definitions tend to be inflexible when new treatments appear. In the United States, most would not use a definition of serious to limit reproductive services, carrier screening, or neonatal intensive care; outside the United States, more supported limitations. There is not sufficient consensus among experienced genetics professionals to define serious genetic conditions for purposes of law or policy. Responses point to social and economic influences on perceptions of what constitutes a serious genetic disorder. © 2002 Wiley-Liss, Inc.