Chronic lung disease is one of the most common causes of death in the world and is particularly high in low income countries. In spite of this there is very little research on the subject and almost none in low income countries that uses adequate quality assurance for the measurements of lung function. The baseline BOLD study measured lung function and collected information on the background, exposures, symptoms, quality of life and treatment of more than 31,000 people in 44 sites in 36 countries. The results of the study so far have shown that many of the assumptions that have been made about chronic lung disease in these countries (and in high income countries also) may be wrong. The prevalence of narrowing of the airways (obstructive spirometry) alone does not explain the high mortality estimated for these diseases in low income settings. A more likely explanation for the high mortality ascribed to chronic lung disease is the smaller size of lungs (restrictive spirometry). This pattern of lung function has largely been ignored for the last half century but a high mortality in those with low lung volumes has been observed in the United States and can explain the higher mortality seen in African Americans who also have smaller lungs on average than white Americans. Although there is no direct evidence for this at the current time, this seems to be the most likely explanation for the high recorded mortality from COPD in South Asia, South-East Asia and Sub-Saharan Africa. Understanding whether this is the case or not is, however, critical for deciding the emphasis of future research and future health policy as obstructive and restrictive diseases have different risk factors and will require different preventive and treatment strategies. This proposal, which will follow up 15,000 people in 20 sites in 15 low or middle income countries, will answer four important questions. First, how far the high mortality from Chronic Respiratory Disease is associated with narrow airways (obstructive disease) or small lungs (restrictive disease). Second how far the low level of lung function found in older people in poorer countries is due to changes in early life that they have never recovered from, or whether their lung function continues to decline more rapidly in later life. Third whether people with poor lung function in low income countries die from respiratory failure or whether the increased risk of death is also due to a higher mortality from heart disease and related problems, as seems to be the case in richer countries. Finally the study will investigate a number of possible risk factors that we believe are important for determining lung health in low income countries focussing particularly on diet and exposure to unregulated industries. Although smoking is by far the most important risk factor for chronic obstruction of the airways there are countries that have been exposed to very little tobacco and yet still have substantial problems with respiratory disease. Other risk factors that have been suggested such as smoke from cooking fires do not seem to explain this discrepancy. By collecting more detailed information in a highly standardised way and by investigating the effects of these and other risk factors on how they influence the rate of decline in lung function over time we will be able to improve considerably our ability to interpret the associations (and sometimes the lack of associations) that we have found in the earlier study. The centres included in this study cover a wide variety of environments and populations in East Asia, South Asia, Central Asia, North Africa, Sub-Saharan Africa and the Caribbean. All of the centres have shown that they are able to collect high quality data, and the co-ordinating centre at the National Heart and Lung Institute in London has a long history of managing large international studies of lung disease. An international advisory board will help to optimise dissemination of results.