Patients with cystic fibrosis (CF) and primary ciliary dyskinesia (PCD) have been shown to have impaired large airway clearance of radiolabelled particles as measured by external gamma camera scanning up to 6 hours post deposition. Recent studies suggest that 24-hour retention of particles may reflect some airway retention in addition to alveolar retention. In a retrospective study, we analyzed the relationship between the deposition pattern and 24-hour retention (Ret24 hr) of technetium 99 radiolabelled iron oxide (99Tc-Fe2O3) particles in 20 patients with CF, 12 patients with PCD, and 17 normal subjects. By gamma camera analysis, initial aerosol deposition was analyzed in terms of central peripheral (C/P) activity within the lungs. Gamma camera scanning was performed immediately following deposition and again at 24 hours to assess residual retention (Ret24hr) as a percent of initial deposition. C/P analysis was also performed on the 24-hour scan (C/P24). For all subjects, initial deposition pattern (C/P) was inversely related to lung function (forced expiratory volume in 1 second \[FEV1]%pred vs. C/P, r =-.54). Ret24hr was also inversely related to initial deposition pattern for all subjects (Ret24hr vs. C/P ratio, r=-.42). Analysis of covariance showed that for a given C/P ratio, CF patients had significantly greater Ret24hr compared to normal subjects (9.8 +/- 2.8\[SE]%). In addition, the CF patients had similar C/P24 as the normal subjects (1.35 +/- 0.40\[SD] vs. 1.10 +/- 0.39, respectively). These results suggest that small airway clearance is compromised in CF patients compared to normal subjects. On the other hand, PCD patients had C/P24 similar to their initial deposition C/P ratios (2.78 +/- 1.72 vs. 2.45 +/- 0.87, respectively), significantly greater than 1.0, and significantly greater than CF or normal subjects, suggesting that PCD patients have prolonged particle retention associated with their large bronchial airways.