THE PROBLEMEpidemiologic evidence relating lung cancer to cigarette smoking has come from two types of study-retrospective and prospective. In the retrospective type, smoking habits and other factors in the histories of patients dying with bronchiogenic carcinoma are reviewed. Such studies have shown a number of variables associated with carcinoma of the lung, but the most startling correlation has been with heavy cigarette smoking. Prospective studies involve the continuing observation of healthy subjects to ascertain the cause of death and the association, if any, with various degrees of smoking. Two sizable prospective studies have been made, viz, that of Hill and Doll (1) on 35,000 British physicians, and that of Hammond and Horn (2,3) on 187,983 white men between the ages of 50 and 69. In both studies, the frequency of death from lung cancer among nonsmokers was strikingly lower than among heavy smokers.