Asthma and chronic obstructive pulmonary disease are both common diseases, which together afflict approximately 25 million Americans. Although expiratory airflow obstruction is the common physiologic abnormality, asthma and chronic obstructive pulmonary disease are characterized by unique pathologic findings, clues from clinical histories, and laboratory test results. Despite some overlap in these characteristics, it is usually possible to differentiate these two conditions. This distinction is important for the healthcare provider to communicate a realistic prognosis to the patient and the patient's family, and to institute appropriate therapy.