The implications of recent demographic trends in developed countries are considered.  The emphasis is on the increase in life expectancy, and particularly in the rate of growth of the numbers of the very old (those aged 85 and over).  "To evaluate the impact of recent mortality reductions on the social security and health service systems of developed countries [the author analyzes] the mortality conditions of 11 developed countries over the period 1950 to 1978."  The countries concerned are the United States, Canada, Japan, Norway, Sweden, Denmark, Czechoslovakia, Hungary, the United Kingdom, the Federal Republic of Germany, and France.  "The results of [the] analyses show that major increases in life expectancy have occurred at advanced ages for females and that the cross-country differences in the cause of death structure indicate that advances were achieved through a variety of mechanisms.  Thus, it appears that no single uniform model of biological aging will currently explain cause specific mortality trends in countries with historically high life expectancies.  This implies that further mortality reductions are possible in these countries by achieving cause specific mortality reductions observed to have occurred in another country."  This is a revised version of a paper originally presented at the 1983 Annual Meeting of the Population Association of America (see Population Index, Vol. 49, No. 3, Fall 1983, p. 413).