With the aging of populations in developing countries there is both a demographic and an epidemiological transition which affects the impact of chronic degenerative diseases on the health status of the populations. Demographic transition takes place in countries where there are effective programmes of disease control which allow for survival during the early years of childhood and adolescence. This results in an increase in life expectancy which places larger proportions of the population in the age range (60 years and older) in which chronic degenerative diseases become the major determinants of health status. Epidemiological transition in diseases may also be brought about by shifts in social and economic patterns which favour detrimental changes in risk factors for the chronic degenerative diseases. Such changes may include health-related behaviour which augments dietary consumption of fats and alcohol, increases obesity, increases smoking and decreases physical activity. Such changes in risk-factor levels increase the prevalence of chronic degenerative diseases which manifest themselves at later ages, and for which early preventive actions could be cost-effective. In order to illustrate the impact of both demographic and risk-factor effects, analyses are made of the impact of increases in life expectancy on cause-specific mortality in both developing and developed countries. It is shown that there is great similarity in the effect of major noncommunicable diseases on the life expectancy of adults in both developed and developing countries. The major differences are seen to be in the proportions of deaths expected from such diseases as cancer, diabetes, heart disease, stroke and cirrhosis; but not in the distribution of age at death which is the better measure of disease impact. Demographic analyses, computing indirect estimates of mortality, also demonstrate that there are currently more chronic disease deaths in developing than developed countries and that as expectation of life increases in developing countries the global chronic disease burden will be greatly concentrated in the developing countries. Analyses of risk-factor reduction by feasible intervention strategies, e.g. smoking cessation campaigns, treatment of high blood pressure, using relationships between risk factors and diseases established in longitudinal studies carried out in developed countries, point out that the effect of risk-factor control in long-living populations can be hidden by the dependency of risk factors and various related causes of death, e.g. smoking has an impact on lung cancer, ischaemic heart disease and emphysema, but at different ages.(ABSTRACT TRUNCATED AT 400 WORDS)