This paper analyzes changes in sex mortality ratios between 1979 and 1987 for adults in 23 developed countries. (A sex mortality ratio is the ratio of male to female death rates.) Previous analyses have shown that during the mid-twentieth century sex mortality ratios increased for all adult age groups. During the 1980s sex mortality ratios continued to increase for 25-34 year olds, but showed mixed trends for other adult age groups. For example, for older adults aged 55-64, sex mortality ratios increased in Southern and Eastern European countries and Japan, but sex mortality ratios decreased in Northern European and Anglophone countries. Trends in several causes of death contributed to these trends in sex mortality ratios. For example, for 25-34 year olds, increases in men's suicide rates and HIV or AIDS mortality contributed to the increases in sex mortality ratios. For older adults, it was hypothesized that decreasing sex differences in cigarette smoking in recent decades would result in decreasing sex differences in lung cancer and ischemic heart disease mortality during the 1980s. The predicted decrease in sex differences in lung cancer mortality was observed in many countries; women had more unfavorable lung cancer mortality trends than men in the Anglophone countries and Northern and Central Western European countries. In contrast, very little evidence was found for the predicted decrease in sex differences in ischemic heart disease. The paper presents additional data concerning the contributions of trends in specific causes of death to changes in sex mortality ratios and briefly reviews evidence concerning probable causes of the observed mortality trends. It appears that recent trends in sex mortality ratios have been influenced by changing sex differences in smoking and a variety of additional factors, such as the effects of improvements in health care interacting with inherent sex differences in vulnerability to ischemic heart disease.