The aim of this study was to determine particle clearance and retention from non-alveolated airways of 14 healthy subjects (HS), 10 subjects with asymptomatic bronchial hyperresponsiveness (BHR), and 23 patients with chronic obstructive pulmonary disease (COPD). Monodisperse iron oxide particles of 1.6 micro m geometric and 3.5 micro m aerodynamic diameter labeled with (99m)Tc were delivered to the airways by inspiration of small aerosol boli into shallow volumetric lung depths. In each subject the penetration front depth of the aerosol boli was adjusted to 55% of the Fowler dead space of the airways. Particle deposition was enhanced by about 7 seconds of breath-holding after bolus inhalation. Retention of the particles in the airways during the 48 hours after their administration was assessed by measuring the decline in lung activity with a sensitive gamma counter. Particle deposition was not significantly different among study groups. Twenty-four hour particle retention in the airways was not different among study groups. Sixty-one percent of the particles were retained at 24 hours in HS, 58% in BHR, and 64% in COPD. However, subjects with BHR showed accelerated mucociliary clearance compared to healthy subjects, whereas clearance was retarded in COPD patients. This long-term particle retention in the airways has to be taken into account in aerosol toxicology risk assessment and aerosol therapy dose evaluation.