It is increasingly recognized that developing countries are undergoing an epidemiologic transition similar to that which occurred in industrialized countries in previous centuries. While infectious diseases are still the main cause of morbidity and mortality, there is a marked increase in chronic non-communicable diseases, particularly in the most advanced developing countries, and these diseases are expected to take the lead in a decade or two. Most of these diseases, above all coronary heart diseases, stroke and diabetes, are related to diet and lifestyles, for example tobacco and alcohol consumption. As a matter of fact, these societies are also facing a growing epidemic of overweight and obesity, due to the frequent energetic imbalance between energy-dense food consumption and reduced daily physical expenditure. This health transition, favoured by demographic changes towards aging populations, is occurring at an increased pace in urban societies widely exposed to the modernization of lifestyle, sedentary occupation, and to lipid- and sugar-rich food, often poor in fibre and micronutrients. Increased world access to cheaper vegetable oil is thought to have triggered off this accelerated and generalized trend, though animal food, rich in saturated fat, and imported or locally-made industrialized food also play a role. While increased national and household incomes facilitate the initial change, as the transition advances poor people progressively become the main victims, as has been observed in the more advanced developing countries. Metabolic imprinting due to intra-uterine and infant malnutrition, which are still common in these societies, is also thought to play a significant role in the increase in the expression of insulin resistance, obesity and chronic diseases when these children are exposed to abundant food and modern lifestyle, later in life. Treatment and secondary prevention of nutrition-related chronic diseases and associated disabilities have an ever rising cost in industrialized countries, which is far beyond the means of the still fragile economies of developing countries. This double burden of infectious diseases and undernutrition that still exist, and of non-communicable diseases and overnutrition represents a threat to the frequently unprepared health care services in developing countries. There is a clear need to focus health policies on the prevention of chronic diseases through primary health care services, the use of mass media for communication and education about healthy nutrition and lifestyle, and the adaptation of public policies. Nutritionists must also adapt to this changing nutritional situation which may result in apparently contradictory nutritional status findings within societies if not even within households.