The efficiency of delivery of an aerosol to 22 children with cystic fibrosis, aged 8 months to 17 years, was investigated with the use of a standard ultrasonic nebulizer. The study was designed to simulate clinical conditions as closely as possible. Retention of the aerosol was monitored with a scintillation camera interfaced to a small digital computer. The children also had ventilation studies with xenon-133, which allowed comparison of the distribution of aerosol with regional ventilation. The aerosol was visualized within the lungs of all 22 patients. Between 0.8 and 6.5 per cent of the initial activity was deposited in the lungs, representing an average of 0.27 ml. per therapy session or 0.013 ml. per kilogram of body weight. There was a positive correlation (r=0.89; p<0.01) between increasing age and greater pulmonary deposition of the aerosol. There was decreased aerosol entry to regions which were poorly ventilated. The efficiency of aerosol deposition is variable in children with cystic fibrosis, depending primarily on the age of the patient, whereas its distribution within their lungs is related to regional variations in the severity of the pulmonary diseases.