Chronic obstructive pulmonary disease (COPD) and asthma are the major causes of pulmonary disability in the United States, with at least 10 million Americans suffering form COPD and up to 5% of the population afflicted with asthma. Over the past 20 years, major strides have been made in our understanding of the pathophysiology of these disorders, although there are still large gaps in our knowledge. While a number of position papers and statements have been promulgated by the American Thoracic Society concerning various aspects of the diagnosis and treatment of COPD and asthma, it was felt that a review of the overall topic was timely. This statement represents the combined efforts of a Task Group appointed by the Scientific Assemble of Clinical Problems of the American Thoracic Society to accomplish this task. Clearly, we could not cover every aspect of this broad topic nor even provide a detailed review of those areas addressed. We elected instead to concentrate on clinically relevant topics and to provide sufficient data to be useful as a guide as well as to include selected, but in no was exhaustive, references. The first two chapters define the entities and set forth recommendations for diagnosis, hospital admission, and discharge. The remaining four chapters critically review the various facets of therapy. We have noted controversial areas and those were conclusive experimental data are not yet available. In these situations, the committee often decided to take a position on one side or the other based upon the best available information.