Between November 1995 and October 1996, particulate matter concentrations (PM10 and PM2.5) were measured in 25 study areas in six Central and Eastern European countries: Bulgaria, Czech Republic, Hungary, Poland, Romania and Slovak Republic. To assess annual mean concentration levels, 24-h averaged concentrations were measured every sixth day on a fixed urban background site using Harvard impactors with a 2.5 and 10 mum cut-point. The concentration of the coarse fraction of PM10 (PM10-2.5) was calculated as the difference between the PM10, and the PM2.5 concentration. Spatial variation within study areas was assessed by additional sampling on one or two urban background sites within each study area for two periods of 1 month. QA/QC procedures were implemented to ensure comparability of results between study areas. A two to threefold concentration range was found between study areas, ranging from an annual mean of 41 to 98 mug m(-3) For PM10, from 29 to 68 mug m(-3) for PM2.5 and from 12 to 40 mug m(-3) for PM10-2.5. The lowest concentrations were found in the Slovak Republic, the highest concentrations in Bulgaria and Poland. The variation in PM10 and PM2.5 concentrations between study areas was about 4 times greater than the spatial variation within study areas suggesting that measurements at a single sampling site sufficiently characterise the exposure of the population in the study areas. PM10 concentrations increased considerably during the heating season, ranging from an average increase of 18 mu gm(-3) in the Slovak Republic to 45 mu gm(-3) in Poland. The increase of PM10 was mainly driven by increases in PM2.5; PM10-2.5 concentrations changed only marginally or even decreased. Overall, the results indicate high levels of particulate air pollution in Central and Eastern Europe with large changes between seasons, likely caused by local heating. (C) 2001 Elsevier Science Ltd. All rights reserved.