The object of this study was to investigate the relationship between residence, occupation and smoking habits, and mortality from chronic diseases, particularly lung cancer. It was a prospective study, initiated by a questionnaire sent to Canadian veteran pension recipients. The study was based on the replies of 78,000 males and 14,000 females, together with data on the deaths occurring among these respondents over a six-year follow-up period-July 1, 1956 to January 30, 1962.The outstanding finding of this study was that cigarette smokers compared to non-smokers had excessive mortality, particularly from heart and circulatory diseases, lung cancer, and bronchitis and emphysema. The mortality ratios for heart and circulatory diseases were elevated even for those who smoked cigarettes less than five years, and remained relatively constant as the duration of smoking increased. The mortality ratios for lung cancer increased markedly as the duration of smoking increased. A small excess in mortality was noted among urban residents. An association between cause of death and occupation was not evident in this study.Findings based on the data on smoking collected in this study were incorporated into the Report of the U.S. Surgeon-General's Advisory Committee on Smoking and Health.