The 1972 U.S. surgeon general's report The Health Consequences of Smoking was the first to include a warning about exposure to second-hand smoke. Because the tobacco industry has a record of withholding the results of their research from the public, we searched the internal tobacco industry documents and compared internal industry research on second-hand smoke to what the industry published in the open scientific literature through 1972. We found chemical analyses, sensory evaluations, and discussions of sidestream cigarette smoke (the smoke emitted by the cigarette between puffs, the main component of second-hand smoke), beginning in 1929. American Tobacco Company research in the 1930s indicated that, compared with mainstream smoke, sidestream smoke was produced in larger quantities and contained, per cigarette, 2 times more nicotine and 12 times more ammonia. Research funded by the Tobacco Industry Research Committee in the 1950s revealed that sidestream smoke contained, per unit cigarette, higher concentrations of carcinogenic polycyclic aromatic hydrocarbons, per unit mass, including four times more 3,4 benzopyrene. In 1956 and 1957, respectively, Philip Morris and R. J. Reynolds also began to research sidestream smoke. In 1961, Philip Morris began to do sensory evaluation and modification of sidestream odor during product development. This sensory evaluation of sidestream smoke was the first biological testing of sidestream smoke by a tobacco company. Prior to the release of the 1972 U.S. surgeon general's report, the tobacco industry published the majority of its findings in the open scientific literature and does not appear to have perceived second-hand smoke as a threat to human health.