One hundred seventy-two preschool children, aged three to six years, who attended municipal day-care centers in central and suburban areas of Helsinki, were followed up for seven weeks during the winter season and for eight weeks during the spring season in 1991. For each child, the weekly average NO2 exposure was estimated using passive samplers attached to the outer garments of the children during their everyday activities. Respiratory symptoms were recorded in daily diaries by the parents. The median of personally measured seasonal NO2 exposures was 21 micrograms/m3 (range 11-45.8 micrograms/m3). The seasonal median NO2 exposure was significantly larger (p < 0.001) in the central area (27.4 micrograms/m3) than in the suburban area (18.2 micrograms/m3), reflecting a difference in exposure caused by automobile traffic. There also were significantly (p < 0.001) more days with stuffed nose (26% vs. 20%) and cough (18% vs. 15%) in the central area than in the suburban area. At the individual level, there was a nonsignificant positive correlation between personal NO2 exposure and cough during the spring season. Thus the areal difference in these symptoms was partly, but not completely, due to the difference in exposure to NO2.