This paper examines the effects of tobacco smoking on the sex mortality differential in the United States. It is found that all forms of smoking combined account for about 47 percent of the female-male difference in 50e37 (life expectancy between ages 37 and 87) in 1962,and about 75 percent of the increase in the female-male difference in 50e37over the period 1910–62. When these percentage effects of smoking are decomposed each into a sum of contributions by age and immediate medical cause of death, the degenerative diseases acting at the older ages are found to be of primary importance. The above results appear in large part to explain why the degenerative diseases also account for most of the 1910–65 increase in the female-male difference in life expectancy at birth. The analysis assumes that spurious effects due to the correlation of tobacco consumption with other mortality-related factors are small compared to the causal effects of tobacco consumption itself.