Sex differentials in life expectancy at birth in Japan are analyzed for the period 1920 to 1990. The results show that there was a general increase in differences in mortality by sex over time. "The sex differential in age-specific death rate in 0-4 year age group (particularly age 0) explained most of the sex differential in life expectancy at birth before 1947. After 1950, the age group of 60-79 played a major role in the sex differential in life expectancy at birth. It is noteworthy that female mortality exceeded male mortality in age groups of 2-41 before 1930. Consequently, excess of female mortality reduced the sex differential in life expectancy at birth at that period. As for the sex differential in mortality rates by causes of death, tuberculosis, pregnancy and childbirth related disease exerted a great influence...before 1940. Recently, malignant neoplasms, heart diseases, cerebrovascular diseases, and accidents [have] become leading contributors to the sex differentials in life expectancy at birth." (SUMMARY IN ENG)