Important divergences in the definition of bronchial asthma, chronic bronchitis and pulmonary emphysema have persisted until the middle 1950's. Definitions were first standardized in 1958 by a group of British experts (CIBA symposium) and made clearer by the WHO Experts Group on Cor Pulmonale in 1961 and by the American Thoracic Society in 1962. The standardized questionnaire proposed by the British Medical Research Council has been widely adopted. For pulmonary emphysema, a precise definition based on anatomical data (dilatation of terminal air spaces with destruction of their walls) has been accepted. Chronic bronchitis is defined in clinical terms (almost daily productive cough 3 months each year, at least on 2 consecutive years; the terms "chronic cough" and "recurrent bronchitis" are still controversial. The greatest difficulties have been encountered in the definition of bronchial asthma; despite a second CIBA symposium (1971) devoted to this problem, it has not been possible to improve on the fairly wide physiological definition put forward in 1958 (bronchial obstruction that varies either spontaneously or under the influence of treatment). For about 15 years peripheral airways obstruction (PAO), sometimes called "small airways disease" has been recognized as a special anatomical and functional syndrome.