The contribution of smoking to sex differences in mortality is estimated on the basis of data from 12 studies of the mortality of nonsmoking men and women, together with mortality data for comparable general population samples. Most of the data are for samples drawn from the U.S. population from the late 1950s to 1980. The findings from different studies are generally consistent, once methodological factors are taken into account. The findings indicate that, for total mortality, the proportion of sex differences attributable to smoking decreases from about two-thirds at age 40 to about one-quarter at age 80. Over the adult age span, it appears that about half of the sex difference in total mortality is attributable to smoking. Findings for ischemic heart disease mortality show a similar pattern. For lung cancer, it appears that about 90 percent of the sex difference in mortality is attributable to smoking. The estimated contributions of smoking include both the effects of sex differences in smoking habits and the effects of sex differences in the increase in mortality caused by smoking. The quantitative results should be interpreted with caution, since several lines of argument suggest that multivariate analyses controlling for other relevant factors would produce lower estimates of the contribution of smoking to sex differences in mortality. Despite this limitation, the findings analyzed in this review, together with additional evidence from related research, strongly support the conclusion that cigarette smoking makes a major contribution to men's higher mortality, but other factors also play an important role.