Conducting airways less than 2 mm in internal diameter were counted and measured in the lungs of patients with and without chronic bronchitis, and with and without emphysema. Eighteen patients without bronchitis or emphysema provided normal values. Eleven patients with chronic bronchitis but no emphysema, none of whom had pulmonary dysfunction, had organic narrowing of airways but no loss of airways. These patients also had more mucus in their small airways. Patients with emphysema had narrowing and slight loss of airways, and there was an exaggerated amount of mucus in their small airways. Fixation of lungs via the bronchus diminished the amount of mucus in airways. It was concluded that peripheral mucus plugging is likely a significant feature of chronic bronchitis and is considerably exaggerated in patients who also have emphysema. Mucus plugging plays a significant part in airway obstruction in patients with emphysema.