Quiet breathing by mouth of a 3-mum aerosol was compared with intermittent positive pressure delivery of the same aerosol to the lung in a group of bronchitics. The pattern of lung deposition and the total dose delivered to the lung were measured. The differences between quiet breathing and intermittent positive pressure breathing were found not to be significant (P greater than 0.5). In addition, during similar or greater total ventilation, intermittent positive pressure breathing delivered a mean of 32% less aerosol to the lung than did quiet breathing. These findings suggest that intermittent positive pressure breathing as currently used for aerosol delivery cannot be expected to enhance peripheral deposition of inhaled aerosols in patients with airway obstruction.