Simultaneous indoor and outdoor PM10 and PM2.5 concentration measurements were conducted in seven primary schools in the Athens area. Both gravimetric samplers and continuous monitors were used. Filters were subsequently analyzed for anion species. Moreover ultrafine particles number concentration was monitored continuously indoors and outdoors. Mean 8-hr PM10 concentration was measured equal to 229 +/- 182 microg/m3 indoors and 166 +/- 133 microg/m3 outdoors. The respective PM2.5 concentrations were 82 +/- 56 microg/m3 indoors and 56 +/- 26 microg/m3 outdoors. Ultrafine particles 8-h mean number concentration was measured equal to 24,000 +/- 17,900 particles/cm3 indoors and 32,000 +/- 14,200 particles/cm3 outdoors. PM10 outdoor concentrations exhibited a greater spatial variability than the corresponding PM2.5 ones. I/O ratios were close or above 1.00 for PM10 and PM2.5 and smaller than 1.00 for ultrafine particles. Very high I/O ratios were observed when intense activities took place. The initial results of the chemical analysis showed that SO4(-2) accounts for the 6.6 +/- 3.5% of the PM10 and NO3(1) for the 3.1 +/- 1.4%.The corresponding results for PM2.5 are 12.0 +/- 7.7% for SO4(-2) and 3.1 +/- 1.9% for NO3-. PM2.5 SO4(-2) indoor concentrations were highly correlated with outdoor ones and the regression line had the largest slope and a very low intercept, indicative of no indoor sources of fine particulate SO4(-2). The results of the statistical analysis of indoor and outdoor concentration data support the use of SO4(-2) as a proper surrogate for indoor PM of outdoor origin.