['To determine if mucus removal is impaired in children with cystic fibrosis (CF) who have been recently infected with Pseudomonas aeruginosa.', 'We compared mucociliary clearance (MCC), cough clearance (CC), lung morphology, and forced expiratory volume in 1 second (FEV1) in 7- to 14-year-old children with CF and mild lung disease (FEV1 ≥ 80%). Children were either P. aeruginosa negative (n = 8), or P. aeruginosa positive (P. aeruginosa obtained from at least 1 airway culture in the preceding 18 months) (n = 10). MCC and CC were quantified from gamma camera imaging of the right lung immediately after inhalation of (99m)technetium sulfur-colloid (time 0), over the next 60 minutes (average percent clearance over the first 60 minutes [AveMCC60]), 60-90 minutes (average percent clearance between 70 and 90 minutes [AveMCC/CC90]), and after 24 hours (percent clearance after 24 hours [MCC24hrs]). Children coughed 30 times between 60 and 90 minutes. Lung morphology was assessed by high resolution computed tomography (HRCT) scores of both lungs (total score) and of the right lung, using the Brody scale. Percent AveMCC60, AveMCC/CC90, MCC24hrs, FEV1, and HRCT scores were compared across the 2 groups using unpaired t tests. Associations were assessed using Spearman correlation.', 'There were no differences between the 2 groups in AveMCC60, MCC24hrs, mean HRCT total scores, right lung HRCT scores, or mean FEV1. AveMCC/CC90 was significantly decreased in children with P. aeruginosa compared with those without (16.2% ± 11.0% vs 28.6% ± 7.8%, respectively; P = .016). There was a significant negative correlation of AveMCC60 and AveMCC/CC90 with total lung HRCT score (all P < .05) but not with FEV1.'] Is mucus removal impaired in children with cystic fibrosis who have been infected by Pseudomonas aeruginosa?