In 1982 the American Cancer Society (ACS) enrolled over 1.2 million American men and women in a prospective mortality study of cancer and other causes in relation to environmental and life-style factors. Biennial follow-up is planned through 1988. At the time of enrollment, 23.6% of the men and 20.0% of the women were current smokers of cigarettes. Compared with a similar ACS study of 1 million subjects enrolled 23 years earlier, among men the proportion of current smokers was halved and that of ex-smokers doubled, while among women the proportion of ever-smokers increased by 10% and that of ex-smokers quadrupled. Most smokers of filter cigarettes had smoked nonfiltered cigarettes earlier in life. The median year for switching to filters was 1964, the year of the first Surgeon General's report. More than one-third of male smokers' and one-half of female smokers' current brands had tar yields below 12 mg; less than 9% of male smokers' and 4% of female smokers' current brands had tar yields of 20.2 mg or more (nonfilters). The study population differed in many respects from the general U.S. population; the study population had, for example, a much higher average educational level. Nevertheless, distributions of smoking habits changed a few percentage points after adjustment to the educational level of the general population.