The scope of this work was to characterize PM mass and number concentration at typical residential microenvironments in the centre of Athens and to examine the relative contribution of the indoor and outdoor sources. Three residential flats located in densely populated residential areas were studied, during a warm and cold period of 2002. PM10, PM2 and black carbon (BC) mass concentrations, as well as ultrafine and accumulation mode particle number size distributions were recorded indoors and outdoors simultaneously. Outdoor concentrations of all size fractions were significant, and indicative of urban sites affected by heavy traffic. Indoor levels were generally lower than the corresponding outdoor ones. Nevertheless, elevated indoor concentrations were recorded, caused by increased ambient air penetration in the indoor microenvironments and/or indoor particle generation. The mean 24-hr indoor PM10 concentration at all residences was 35.0 ± 10.7 μg/m3 during the warm period and 31.8 ± 7.8 μg/m3 during the cold period. The corresponding PM2 concentration was 30.1 ± 11.1 μg/m3 and 27.2 ± 3.6 μg/m3 during warm and cold periods, respectively. Regression analysis of indoor and outdoor concentration data revealed that indoor BC may be considered mainly of outdoor origin. A large fraction of the outdoor-generated PM2 and ultrafine and accumulation mode particles also seems to penetrate indoors, causing elevated indoor levels. Regarding indoor particle generation, cooking was the strongest contributor in residential microenvironments.