The course of mortality has undergone some remarkable changes since the early part of the last century, the period for which official statistics are available. The variations in the death-rates are emphasised when the rates of various countries are compared, and the cause of the difference in the trend of mortality between various countries has been the source of much speculation. The most striking contrast between the course of mortality in two countries is, probably, that discussed by Prof. Greenwood (1924) in his paper, “The vital statistics of Sweden and England” (J. Roy. Stat. Soc. 87, Part 4). He made a comparison between the two countries for the ages 10–50 for two periods. The first period covered the years 1838–54 for England and Wales and 1841–50 for Sweden, the second period was 1901–10 for both countries. The probability of dying for males during the first period was greater in England and Wales between the ages 10–35 than in Sweden. The greatest difference occurred at age 19, when the value for England and Wales was 151·2 per cent, of the Swedish figure. From age 36 the male mortality in Sweden was in excess of that for England and Wales. The female mortality was greater in England and Wales for every age 10–50. In the second period the mortality at adolescence in England and Wales had made such an improvement that it was less than that of Sweden, the probability of dying for males in England and Wales being less than that of Sweden for the ages 10–31. The England and Wales value at age 19 was now only 59·8 per cent, of that of Sweden. From age 32 the male mortality in Sweden was less than that of England and Wales. The female rate for England and Wales was below that of Sweden for the ages 10–36 and above for 37–50. The mortality of 1901–10, of both countries, had shown considerable improvement on that of the earlier period. This decline in mortality has been general and has been shown in the experience of most countries.