1. The distribution of 5 μm 99m Tc aerosol particles deposited in the lungs was compared with that of 81m Kr gas in 20 asthmatic subjects (FEV 1.0 ≥75% predicted) whose asthma was mild or in remission. Sixteen normal non-smokers and 14 ‘normal’ cigarette smokers were studied as control subjects. 2. Radioaerosol lung penetration in the ‘normal’ smokers was the same as in the normal non-smokers but was reduced in the asthmatic subjects. 3. Radioaerosol penetrated better than 81m Kr to the lung apices of normal non-smokers and asthmatic subjects; it penetrated better than 81m Kr to the lung bases of the asthmatic subjects. 4. Uneven distribution of aerosol relative to 81m Kr related strongly to maximal mid-expiratory flow rate (MMFR) in the asthmatic subjects. 5. Aerosol deposition abnormalities in symptom-free asthmatic subjects may reflect basal airways closure at functional residual capacity and abnormal patterns of lung filling.