When we were invited to prepare this background paper on the health problems of the developing countries for the Commission on Health Research for Development, our first thought was to compile and organize available data on the causes of morbidity and mortality affecting different age groups in various populations. It soon became clear that this would not be especially useful. There are major gaps in the available data, particularly from the poorer countries and for people above 5 years of age. The data that are available are often of poor or uncertain quality, collected from unrepresentative or undefined subpopulations, and not strictly comparable due to different definitions and data-collection methods. Additionally, in the absence of agreed definitions and analytical frameworks, it is not clear what could or should be done with the data on health problems so amassed. More fundamentally, we have come to doubt whether the current array of epidemiological concepts and tools is sufficient for the task. We therefore decided that, while giving an overview of current knowledge on levels and trends of morbidity and mortality, the emphasis of this paper should be more towards concepts, methods, and data deficiencies. In Section 1, we set out definitions and frameworks for considering health problems and health research; we review recent conceptual models for the analysis of the determinants of child survival; and we outline a framework, focusing on modifiable determinants of health and life-cycle health effects, which is used in subsequent sections. In Section 2, relationships between national and societal level determinants and health are reviewed and then set aside. In Section 3, we review available data on world patterns and trends of morbidity and mortality, highlighting the data deficiencies and lacunae. In Section 4, we follow the life of a woman in a developing country and examine the health problems, and their determinants, which she and her children face. In Section 5, we draw these strands together and, having reviewed current approaches to prioritizing health problems and suggested some ways in which they could be improved, in Section 6 identify several research priorities, emphasizing the need for methodological research. This paper was commissioned in March 1987; prepared in draft and presented to a meeting at Chateau de Bossey, Geneva, Switzerland during 15-17 July; and revised and completed in September 1987. It is in no sense definitive or final.(ABSTRACT TRUNCATED AT 400 WORDS)