There has been a rapid increase in the number of deaths from pulmonary emphysema, together with a smaller increase in deaths due to the related diagnoses, asthma, bronchitis, bronchiectasis, and chronic interstitial pneumonia. Principles of nomenclature of these diseases have been reviewed. "Obstructive ventilatory disease of the lung" is suggested as a name for this group of disorders, and criteria for its use have been suggested. If the increasing frequency of these disorders is to be halted and the disease prevented, epidemiologic and clinical studies must be undertaken, using unequivocal criteria for symptoms and laboratory tests. The concept of obstructive ventilatory disease has some important advantages for this purpose when compared with the diagnoses of asthma, chronic bronchitis, or emphysema.