A new model of aerosol deposition in the human lungs has been developed. It incorporates the deposition probability equations of Landahl and Findeisen in the Horsfield Asymmetric Model 1 of the lung. The deposition model takes into account the regional distribution of ventilation by incorporating principles of ventilatory mechanics into the determination of flow distribution in the lung. Calculations are performed for a 4 micrometers aerosol inhaled with a breathing pattern consisting of a 1,000 ml tidal volume and an average inspiratory flow of 500 ml/sec. A ventilation gradient from base to apex of 1.10 is assigned. The results show that deposition by impaction dominates in the large airways, while deposition by sedimentation dominates in the small airways and alveoli. Calculations of surface concentrations of particles deposited in the airways reveal that the segmental and subsegmental bronchi receive the highest concentrations. The gradient of particles deposited per unit lung volume from base to apex equals 1.13 which is very close to the ventilation gradient. The new model is the first attempt to assess the distribution of deposited particles in an asymmetric model of the lung, using a realistic distribution of ventilation.