The history holds the central role in distinguishing among asthma, chronic bronchitis, and emphysema. A personal or family history of atopy, a history of seasonal worsening of disease in response to a known environmental agent, perhaps seasonal, and marked variability in the severity of airflow obstruction, often with dramatic responsiveness to bronchodilator drugs, strongly support the diagnosis of asthma. Exacerbation of wheezing by exposure to cold air or following the ingestion of a drug, and asthma variants, such as nocturnal cough responsive to bronchodilator agents or exercise-induced asthma, all support the diagnosis of asthma. Peripheral blood eosinophilia or sputum eosinophilia support the diagnosis of asthma providing other known causes of eosinophilia can be excluded. Positive skin tests are helpful in establishing the atopic state and indicating its possible etiology. An elevated serum IgE level supports the diagnosis of asthma; a normal one does not exclude it. Cigarette smoking is a common background factor in both chronic bronchitis and emphysema, and both diseases are infrequently observed in the absence of this history. Long-standing mucous hypersecretion preceding airflow obstruction suggests the presence of chronic bronchitis. Progressive dyspnea on effort as the predominant symptom suggests the possibility of emphysema. Reversibility of airflow obstruction suggesting the presence of asthma can be obtained either from physical examination or serial pulmonary function studies. Apart from this, neither of these techniques is very useful in differential diagnosis. Evidence of emphysema in the chest roentgenogram and a low value of the Dco/VA are sensitive tests for the presence of emphysema but are not highly specific. The main value of making the differentiation among these three conditions now lies in establishing a prognosis and guiding the use of corticosteroid therapy. As new information accumulates on the pathogenesis, prevention, and treatment of asthma, chronic bronchitis, and emphysema, precise diagnosis is likely to acquire increased significance.