High concentrations of ultrafine particles have been reported to exist near major freeways. Many urban residences are located in close proximity to high-density roadways. Consequently, indoor environments near freeways may experience significant concentrations of outdoor ultrafine particles. Given that people spend over 80% of their time indoors, understanding transport of ultrafine particles from outdoor to indoor environments is important for assessing the impact of exposure to outdoor particulate matter on human health. Four two-bedroom apartments within 60 m from the center of the 405 Freeway in Los Angeles, CA were used for this study. Indoor and outdoor ultrafine particle size distributions in the size range of 6-220 nm were measured concurrently under different ventilation conditions without indoor aerosol generation sources. The size distributions of indoor aerosols showed less variability than the adjacent outdoor aerosols. Indoor to outdoor ratios for ultrafine particle number concentrations depended strongly on particle size. Indoor/outdoor (I/O) ratios also showed dependence on the nature of indoor ventilation mechanisms. Under infiltration conditions with air exchange rates ranging from 0.31 to 1.11 h(-1), the highest I/O ratios (0.6-0.9) were usually found for larger ultrafine particles (70-100 nm), while the lowest I/O ratios (0.1-0.4) were observed for particulate matter of 10-20 nm. Data collected under infiltration conditions were fitted into a dynamic mass balance model. Size-specific penetration factors and deposition rates were determined for all studied residences. Results from this research have implications concerning personal exposure to freeway-related ultrafine particles and possible associated health consequences. (c) 2004 Elsevier Ltd. All rights reserved.