In an industrial region respiratory morbidity of children was studied in 3 localities with different levels of air pollution within maximal permissible concentrations (average monthly SO2 0.35 mg/m3, 0.25 mg/m3 and less than 0.15 mg/m3 and correlating concentrations of CO, NO2 and particles). It was shown that acute respiratory morbidity and prevalence of recurrent bronchitis did not correlate with SO2 level and was highest in a big city. There was however a correlation with SO2 level of bronchial asthma prevalence (3.0, 2.7 and 2.1), as well as morbidity with acute bronchitis (45.1, 23.3 and 10.6) and acute obstructive bronchitis (15.0, 5.4 and 2.4)--all figures per 1,000 children. Daily acute respiratory morbidity also did not correlate with peaks of SO2 during 3 preceding days, but acute bronchitis correlated with such peaks 2 days before (r = +0.5). Passive smoking and stove heating were significantly more frequent in families of children with respiratory complaints as compared to those without such complaints. Since these differences were more pronounced in industrial towns than in rural areas, it well may be that industrial pollutants potentiate or aggravate the effect of indoor air pollution.