Until the 1960's there was great confusion, both within and between countries, on the meaning of diagnostic terms such as emphysema, asthma, and chronic brochitis. Proposals made by a group of British doctors in 1959 gradually received widespread acceptance but in recent years some new problems have developed. These include difficulties in the definition of airflow obstruction, recognition that what used to be regarded as a single disease, chronic bronchitis, comprises at least two distinct pathological processes, and uncertainty about the degree of variability which distinguishes asthmatic from more persistent forms of airflow obstruction. These are all problems which could be solved by continuance of appropriate research and of riqorous attention to the principles which determine accurate and acceptable definitions of disease.