Ultrafine particle (UFP) exposures have the potential to elicit adverse health effects. People spend most of their time within their place of residence. Little information is available on UFP levels in homes in mainland China. To contribute new data to this important topic, we made time-series measurements of particle number (PN) concentrations and resident activities inside four apartments in high-rise buildings in Beijing during June to August 2009. Indoor PN concentrations at the four sites, averaged over the few-day duration of monitoring at each site, spanned an order of magnitude, from 2800 to 29,100 cm−3. This wide range resulted from differences among apartments in three main factors: (1) the frequency of indoor source events, including cooking activities and intrusion of cooking exhaust from neighboring apartments; (2) the extent of natural ventilation via open windows; and (3) the extent of active air filtration. Daily-integrated PN exposure of the thirteen residents, while in their apartments, ranged from 45,000 to 494,000 cm−3 h/d. For two sites at which outdoor PN concentrations were also measured, the percentage of daily-integrated residential exposure attributable to particles of outdoor origin was 58% for the residents of one site and 81% for residents of the other.