Lung cancer rates and mortality have risen in epidemic proportions in the United States and other industrialized nations during the 20th century. Case-control and cohort studies performed in the 1950s and 1960s firmly established cigarette smoking as the single greatest risk factor for lung cancer. In the United States, overall lung cancer mortality rates in men and women rose progressively from the 1950s. Fortunately, lung cancer incidence and mortality are now declining in middle-aged men. Smoking has significantly increased lung cancer rates among women and is on the rise in developing countries. Environmental agents found in the home and workplace, including radon and asbestos, have also been shown to increase lung cancer risk in both smokers and nonsmokers. Government regulations have helped curtail quantities of these and other atmospheric carcinogens. Efforts to reduce lung cancer risk must be continued and their scope expanded in order to have a global impact on the incidence and mortality of this fatal malignancy.