Objectives: Airborne particle concentrations can be used as quality indicators of indoor environments. The previous lack of reference data has limited the use of particle measurements in offi ce environments. The aim of this study was to describe the concentrations of airborne particles (≥ 0.5 μm and ≥ 5.0 μm) in 122 Finnish offi ce buildings with suspected indoor air problems. Materials and Methods: The database consisted of indoor air and supply air particle samples collected in 2001–2006 from the Helsinki area. The particle concentrations (≥ 0.5 μm and ≥ 5.0 μm) were measured in the indoor air (528 samples from 122 offi ce rooms) and in the supply air (384 samples from 105 offi ce rooms) with an optical particle counter. Airborne particle concentrations ≥ 0.5 μm were categorized according to the effi ciency of supply air fi ltration and health survey data. Results: The mean concentrations in the indoor air equaled 1900 particles/l and in the supply air 1300 particles/l. The effi ciency of supply air fi ltration decreased the fi ne particles counts in both the indoor and supply air. The counts of large particles, ≥ 5.0 μm, were low in the indoor air. Airborne counts of ≥ 0.5 μm particles (geometric mean) were statistically higher in the offi ces whose occupants had work-related symptoms (eye and/or upper respiratory symptoms or upper respiratory infections) than in the offi ces whose occupants had no such symptoms. However, the symptoms may also be linked to other indoor air problems or particle characteristics not studied in this work. Conclusions: This study indicates typical airborne particle levels (≥ 0.5 μm and ≥ 5.0 μm) in Finnish offi ce buildings with suspected indoor air problems. The results can be used to evaluate the quality of indoor environment, possible indoor air problems, and the need for additional investigations.