We collected airborne particles in the air using a high-volume air sampler, and measured chironomid allergen in the air-filter extract by use of RAST inhibition assay. The results obtained were as follows: 1) The assay revealed the presence of chironomid allergen in the air varying from 6.8 to less than 0.3 ng/m3. 2) In Toyama, seasonal fluctuations of airborne chironomid allergen demonstrated some large peaks in summer (June to August), and showed a tendency to decrease in winter. 3) There were significant correlations between airborne chironomid allergen and dry weight of chironomid midges, collected by using light traps, in all period of a week just before, 1 to 2 weeks before, 2 to 3 weeks before, or 3 to 4 weeks before air sampling. There were not significant correlations between airborne chironomid allergen and weathers (temperature, wind speed and precipitation) during or just before air sampling. 4) In Toyama, the frequency of asthma attacks in the asthmatic children with positive C.p. RAST was much increased in summer compared with that of the negative, in which the amount of airborne chironomid allergen was increased. These results show that there is large amount of chironomid allergen in the air from spring to autumn, especially large in summer, and it is related for some asthmatics to the increase of the frequency of asthma attack.