A review of demographic trends and health and social problems in the fast growing urban areas of the world indicates that, in the future, increasing numbers of people will be living in precarious socioeconomic conditions which impede the achievement of health. It is estimated that from 4.4 billion in 1980 the world's population will increase to 6.2 billion by the year 2000. The urban population will increase from 1.8 to 3.2 billion during the same period, over 2 billion of which will be in developing countries. The rapid and often uncontrollable demographic growth of cities, especially in the developing world, stimulates the demand for resources, intensifies their utilization and creates an intolerable pressure on the urban infrastructure and physical environment. A number of action oriented projects to combat disease and contamination have been successful. Projects in Addis Ababa, Ethiopia, Colombo, Sri Lanka, Hyderabad, India, Guayaquil, Ecuador, Lima, Peru, and Rio de Janeiro have been implemented under a partnership among WHO, UNICEF, the Netherlands Aid Agency, the World Bank, and other international organizationals and governments. These projects all emphasize the fundamental role of community organizations, especially that of women; low-cost technology and the need to mobilize and efficiently use locally available resources; an ecological multisectoral concept of health whereby action concerning the environment, education, income generation and the availability of food, all with a powerful disease preventive potential, carry equal if not greater weight than the efforts to provide the population with health centers or implement curative practices. All these projects are focused on marginal groups; many were initiated by imaginative individuals or groups with a considerable amount of social orientation and motivation, and often, at least in the beginning, without the support of governments, nongovernmental or international organizations. It is important to study these projects in their accomplishments and failures; to help describe them and disseminate related information when appropriate; and to promote political and technical support for those which are successful so that they can rapidly come out of the experimental/demonstration phase and be expanded to become part of routine programs.