We studied the effect of bronchial obstruction on central airway deposition of a 0.9% saline aerosol (MMAD = 1.12 micron; sigma g = 2.04) labeled with 99mTc sulfur colloid. Radioaerosol was inhaled on 2 occasions by 8 patients with asthma. The degree of bronchial obstruction at the time of radioaerosol inhalation was measured by the FEV1. Mucociliary clearance of the radioaerosol was used as an index of regional aerosol distribution, because clearance from the densely ciliated central airways occurs more rapidly than from the peripheral, nonciliated regions of the lung. Using the Weibel lung model and an average mucociliary clearance rate of 1 mm/min, we determined that clearance of the radioaerosol from lung generations 1 to 5 (central airways) would be complete within approximately 90 min. Central airway deposition was therefore quantified as radioaerosol clearance in 97 min using a gamma camera. On Days 1 and 2, clearance ranged from 0 to 45% and from 0 to 17%, respectively; FEV1 as a percent of predicted FEV1 ranged from 36 to 88 on Day 1, and on Day 2 from 54 to 92. Radioaerosol clearance was inversely correlated with the baseline FEV1, with r = -0.7673 (linear regression analysis; p less than 0.05). These data suggest that the magnitude of bronchial obstruction is a determinant of aerosol distribution within the lung of patients with asthma and that increased bronchial obstruction enhances central airway deposition of inhaled particles.