Detailed regional deposition of inhaled particles was investigated in young adults (n = 11) by use of a serial bolus aerosol delivery technique. A small bolus (45 ml half-width) of monodisperse aerosols [1-, 3-, and 5-microns particle diameter (Dp)] was delivered sequentially to a specific volumetric depth of the lung (100-500 ml in 50-ml increments), while the subject inhaled clean air via a laser aerosol photometer (25-ml dead volume) with a constant flow rate (Q = 150, 250, and 500 ml/s) and exhaled with the same Q without a pause to the residual volume. Deposition efficiency (LDE) and deposition fraction in 10 local volumetric regions and total deposition fraction of the lung were obtained. LDE increased monotonically with increasing lung depth for all three Dp. LDE was greater with smaller Q values in all lung regions. Deposition was distributed fairly evenly throughout the lung regions with a tendency for an enhancement in the distal lung regions for Dp = 1 micron. Deposition distribution was highly uneven for Dp = 3 and 5 microns, and the region of the peak deposition shifted toward the proximal regions with increasing Dp. Surface dose was 1-5 times greater in the small airway regions and 2-17 times greater in the large airway regions than in the alveolar regions. The results suggest that local or regional enhancement of deposition occurs in healthy subjects and that the local enhancement can be an important factor in health risk assessment of inhaled particles.