Methods. The relationship of cigarette smoking and smoking cessation to mortality was investigated among men screened for the MRFIT, cigarette smoking was an important risk factor for all-cause, coronary heart disease (CHD), stroke, and cancer mortality. These risks, on the log relative scale, were strongest for cancers of the lung, mouth, and larynx. The excess risk associated with cigarette smoking was greatest for death from CHD. Overall, approximately one-half of all deaths were associated with cigarette smoking. Among the 12,866 randomized participants, weak positive associations with duration of cigarette smoking habit and tar and nicotine levels were found with all-cause mortality. For both SI and UC men, substantial differences in subsequent CHD (34-49%) and all-cause (35-47%) mortality were evident for men who reported cigarette smoking cessation by the end of the trial compared with those continuing to smoke. There was no evidence that lung cancer death rates were lower among cigarette smokers who quit compared with those who continued to smoke in this 10-year follow-up period. Conclusion. The data are consistent with results of previous epidemiologic studies indicating that the benefits of smoking cessation on CHD are rapid, while for lung cancer, the benefit is not evident in a 10-year follow-up period.