Epidemiologic studies of uranium miners and other underground miners have consistently shown miners exposed to high levels of radon to be at increased risk of lung cancer. More recently, concern has arisen about lung cancer risks among people exposed to lower levels of radon in homes. The current Canadian guideline for residential radon exposure was set in 1988 at 800 Bq/m(3). Because of the accumulation of a considerable body of new scientific evidence on radon lung cancer risks since that time, Health Canada sponsored a workshop to review the current state-of-the-science on radon health risks. The specific objectives of the workshop were (1) to collect and assess scientific information relevant to setting national radon policy in Canada, and (2) to gather information on social, political, and operational considerations in setting national policy. The workshop, held on 3-4 March 2004, was attended by 38 invited scientists, regulators, and other stakeholders from Canada and the United States. The presentations on the first day dealt primarily with scientific issues. The combined analysis of North American residential radon and lung cancer studies was reviewed. The analysis confirmed a small but detectable increase in lung cancer risk at residential exposure levels. Current estimates suggest that radon in homes is responsible for approximately 10% of all lung cancer deaths in Canada, making radon the second leading cause of lung cancer after tobacco smoking. This was followed by a perspective from an UNSCEAR (United Nations Scientific Committee on the Effects of Atomic Radiation) working group on radon. There were two presentations on occupational exposures to radon and two presentations considered the possibility of radon as a causative factor for cardiovascular disease and for cancer in other organs besides the lung. The possible contribution of environmental tobacco smoke to lung cancers in nonsmokers was also considered. Areas for future research were identified. The second day was devoted to policy and operational issues. The presentations began with a perspective from the U.S. Environmental Protection Agency, followed by a history of radon policy development in Canada. Subsequent presentations dealt with the cost-effectiveness of radon mitigation, Canadian building codes and radon, and a summary of radon standards from around the world. Provincial representatives and a private consultant were given opportunities to present their viewpoints. A number of strategies for reducing residential radon exposure in Canada were recognized, including testing and mitigation of existing homes (on either a widespread or targeted basis) and changing the building code to require that radon mitigation devices be installed at the time a new home is constructed. The various elements of a comprehensive national radon policy were set forth.