During the winter season 2005/2006, measurements of small particulate matter were taken in 22 schools in southwest Germany. PM2.5 was measured gravimetrically. Laser particle counter (LPC) and condensation particle counter (CPC) were used to determine different particle fractions. The investigation included measurements inside (in classrooms) and outside of school buildings. During the whole investigation period continuous reference measurements were taken near the State Health Office in the city of Stuttgart. Information on building-specific data and traffic near the schools was collected via questionnaire. Although distances between the different schools and the location of the State Health Office building in Stuttgart in some cases exceeded 100 km, the concentration levels of particles > 0.3 microm measured by LPC near the different schools were similar to those measured in Stuttgart. The differences between the measurements in Stuttgart and at the other locations were smaller than the variation in time. In the winter season, the impaction of particulate matter was strongly influenced by specific weather conditions. Time resolution of measurements in classrooms showed variation in particle concentration depending on the type of building and indoor activities like cleaning or moving during breaks. Concentrations of very small particles in buildings and in ambient air measured by CPC were influenced by traffic emissions. The observed data give reason to assume that the influence of high traffic emissions to indoor particle concentration may have been overestimated. Furthermore, there is an urgent need for standardization of measurement protocols, sampling, and determination of indoor particulate matter. The classical gravimetric methods are less appropriate since they do not allow for a time resolution of measurements.