To examine the tobacco industry's public and private responses to rising concern over the health effects of environmental tobacco smoke (ETS). Documents from Brown and Williamson Tobacco Corporation (B&W), the British American Tobacco Company (BAT), and other tobacco interests provided by an anonymous source, obtained from Congress, and received from the private papers of a former BAT officer. All available materials, including confidential reports regarding research and internal memoranda exchanged between tobacco industry lawyers. Privately, B&W and BAT began conducting research related to ETS in the mid 1970s. BAT researchers appear to have determined that sidestream smoke produces irritation, that it contains toxic substances including N-nitrosamines, and that it is "biologically active" (eg, carcinogenic) in laboratory tests. During the 1980s, the primary purpose of BAT's research related to ETS was to develop a new cigarette that emitted less irritating and less biologically active sidestream smoke. Publicly, the tobacco industry has denied that exposure to ETS has been proven dangerous to health. It has criticized the methodology of published research on ETS, even when some of its own consultants have privately acknowledged that the research was valid. In addition, the industry has funded scientific research with the stated purpose of anticipating and refuting the evidence against ETS. The tobacco industry's strategy regarding passive smoking has been remarkably similar to its strategy regarding active smoking. It has privately conducted internal research, at least some of which has supported the conclusion that passive smoking is dangerous to health, while it has publicly denied that the hazards have been proven.