Urbanization involves a physical change in which increasing proportions of populations live in urban settings, however defined. It also implies considerable changes in the ways in which these people live, how they earn their livelihoods, the food which they eat, and the wide range of environmental factors to which they are exposed. There is another underlying assumption that, increasingly, urban populations will be more healthy than their rural counterparts and that higher levels of urbanization will equate with better health status. This paper discusses some of the assumptions underlying this contention. It takes issue with certain of them, particularly the assumption that urbanization affects the health of all residents equally. It is manifestly evident that in many cities, particularly in the developing world, the poor are exposed to greater risks and have much lower health status than their richer neighbours. In addition, whilst urban residents may theoretically have a better access to health care and services than do residents in many rural areas, and whilst many indicators of health do appear better in more highly urbanized societies than ones less so, there are caveats. The paper introduces the concept of epidemiological transition, which suggests that, whilst life expectancy might be higher in many urbanized countries and in certain cities, the inhabitants are often merely suffering from different forms of ill-health, often chronic or degenerative, rather than infective. In certain cities in middle-income countries, residents, particularly the poor, are exposed to a double risk of both infection and chronic degenerative ailments. The paper concludes with a consideration of more general recent statements from the World Health Organization among others, on the impact of urbanization on health. The 'Healthy Cities' project is also discussed. WHO identifies a range of general determinants of urban health: physical, social, cultural and environmental. Many represent the by-products of modernization and especially industrialization. It is emphasized that urbanization, and the concentration of human beings into new areas in particular, can bring exposure to new risk factors for large numbers of people. The growth of infectious and parasitic disease in some urban settings must therefore be recognized, as must the emergence of chronic diseases, with the concomitant need for investment in new types of health and social care. However, a number of constraints militate against the achievement of improved urban health, especially in developing countries.(ABSTRACT TRUNCATED AT 400 WORDS)