Evidence from epidemiologic studies linking ambient concentrations of particulate matter to morbidity and mortality influenced the guidelines for air quality standards worldwide. With the improvement of measurement techniques, clearer effects were observed with smaller particle sizes. Based on these effects and results from animal studies on the potential toxicity of ultrafine particles, recent epidemiologic studies focus on the health effects of particles which are less than 100nm in diameter. However, most of the studies are ongoing and only few results have been available so far. Six panel studies with patients suffering from chronic pulmonary diseases have been performed in Germany, Finland and the United Kingdom. Overall, a decrease of peak expiratory flow (PEF) and an increase of daily symptoms and medication use was found for elevated daily particle concentrations. Effects were seen with both fine and ultrafine particles. One large study on daily mortality from Germany showed comparable effects of fine and ultrafine particles in all size classes considered. However, fine particles showed more immediate effects while ultrafine particles showed more delayed effects on mortality. The limited number of epidemiological studies suggest that there are health effects of fine and ultrafine particles which might be independent of each other. If these effects are confirmed by ongoing research, monitoring and regulation of particulate air pollution may need to be revised.