In 1959, a group of British physicians took part in a landmark symposium which defined a number of conditions associated with chronic air-flow obstruction (1). Among these was emphysema, which was defined as enlargement of the acinus that might or might not be accompanied by destruction of respiratory tissue. Subsequently, the World Health Organization (2) and the American Thoracic Society (3) limited the term emphysema to enlargement of any part or all of the acinus accompanied by destruction of respiratory tissue. Destruction was never defined. Despite this, considerable research done in the 1960s led to the recognition of several anatomic forms of emphysema. It also became apparent that patients with some forms of emphysema might be asymptomatic and that the clinical syndrome of air-flow limitation and dyspnea was more closely associated with the severity than the type of emphysema. Centrilobular or panacinar emphysema, alone or in combination, were encountered most frequently. This workshop was convened because of a perceived need to review the current status of the definition and classification of conditions in humans characterized by enlargement and destruction of the respiratory airspaces. There was concern that the term" destruction," which occupies a central place in the American Thoracic Society (3) definition of emphysema, has no clear and agreed upon definition or diagnostic criteria. This lack made it difficult to categorize certain conditions such as the uniform enlargement of respiratory airspaces in Down's syndrome. There was concern about confusion between airspace enlargement and destruction associated with fibrosis and the process associated with clinically significant emphysema, where little fibrosis is generally observed. There was concern that despite the proliferation of animal models of emphysema during the past 20 years, criteria for the development of these animal models have not been agreed upon. Finally, there was concern that contributions to the literature based on studies of human emphysema have decreased sharply in the last decade, despite a rising interest in the study of this dis-