PIP: In the early 1980s, a newborn girl in the developed countries could expect to live many years longer than a boy. The number of additional years of female life expectancy ranged from 5 to 9 years in most of the countries. These large sex differentials in life expectancy reflect the fact that males in developed countries today have higher mortality than females in every age group and for most causes of death. In contrast, early in the 20th century, higher female than male death rates in many age groups were not uncommon, and sex differentials in life expectancy were narrower by several years in most developed countries. This paper presents estimates for the early 1980s of the size of sex differentials in life expectancy in developed countries (which was 6.7 to 6.8 years), and the contributions of age groups and causes of death to those differentials. Diseases of the circulatory system were found to account for nearly 40% of the mean sex differential in life expectancy; neoplasms for 18%; accidents, suicide and violence for 19%, and diseases of the respiratory system for nearly 10%. The 2nd half of the paper examines trends in sex differentials in life expectancy since the late 19th or early 20th centuries, in the context of the transition from high to low mortality. The contributions of age groups and causes of death to changes in the sex differentials between 1900 and the 1980s are estimated for selected countries. The database for the study consists of a set of life tables covering the period from the 19th century to the early 1980s. Around the turn of the century, sex differentials in life expectancy were generally narrower by several years than they are today. Large declines have occurred in death rates from infectious and parasitic diseases in the 20th century. Because male mortality from this group of causes was higher than female mortality, their decline in importance has had a narrowing effect on the sex differential in life expectancy. Declines in female mortality associated with pregnancy and childbirth have contributed to a widening of the sex differential by several tenths of a year. Mortality from neoplasms has increased much more rapidly for males than for females. Mortality from rheumatic fever, for which female mortality was = or than that of males, has decreased. Mortality from cerebro-vascular disease, which affected the sexes nearly equally, has also decreased in importance. Mortality from coronary artery disease, which is more common among males, has increased in importance. Perhaps the factor contributing the most to the widening of the sex differential has been male cigarette smoking leading to elevated mortality from lung cancer and heart disease.