This, the eighth monograph in the Smoking and Tobacco Control series published by the National Cancer Institute (NCI), is in many respects also the most significant. Contained in this volume are new results from five of the world's largest prospective epidemiological studies defining the magnitude of disease risks caused by cigarette smoking. Thirty years ago, in January 1966, NCI published a similar monograph titled Epidemiological Approaches to the Study of Cancer and Other Chronic Diseases. The report of the Surgeon General's Advisory Committee on Smoking and Health had been released in 1964 and had relied extensively on data from prospective mortality studies to delineate the relationship between cigarette smoking and various chronic diseases. The 1966 NCI monograph provided a detailed examination of the outcomes of several of the large prospective mortality studies presented in the 1964 advisory committee report. At that time, the outcomes available from these studies were based on 3 to 6 years of follow up; with the exception of the American Cancer Society's (ACS) Cancer Prevention Study I (CPS-I), studies in the 1966 NCI monograph did not include substantial numbers of females. This monograph includes three new prospective mortality studies (CPS-II, the Nurses' Health Study, and the Kaiser Permanente Prospective Mortality study, provides the outcomes of the CPS-I study after 12 years of follow up, and provides 26 years of follow up of the study of U.S. veterans. Data from these studies provide the most comprehensive description of the disease consequences produced by smoking available to date and are accompanied by a detailed description of the changes in smoking behaviors of the U.S. population over the past century. Prospective mortality studies continue to play a critical role in quantifying the relative mortality risks of smoking for the individual as well as in estimating the overall disease burden caused by cigarette smoking in our society. The goal of this monograph is to facilitate both these tasks by providing, in one volume, comprehensive descriptions of smoking behaviors and the disease risks that result from those behaviors.