It has been suggested that ultrafine particles in urban air may cause the health effects associated with thoracic particles (PM10). We therefore compared the effects of daily variations in particles of different sizes on peak expiratory flow (PEF) during a 57-day follow-up of 39 asthmatic children aged 7-12 years. The main source of particulate air pollution in the area was traffic. In addition to the measurements of PM10 and black smoke (BS) concentrations, an electric aerosol spectrometer was used to measure particle number concentrations in six size classes ranging from 0.01 to 10.0 microns. Daily variations in BS and particle number concentrations in size ranges between 0.032 and 0.32 micron and between 1.0 and 10.0 microns were highly intercorrelated (correlation coefficients about 0.9). Correlations with PM10 were somewhat lower (below 0.7). All these pollutants tended also to be associated with declines in morning PEF. However, the only statistically significant associations were observed with PM10 and BS. Different time lags of PM10 were also most consistently associated with declines in PEF. Therefore, in the present study on asthmatic children, the concentration of ultrafine particles was no more strongly associated with variations in PEF than PM10 or BS, as has earlier been suggested.