Ten children, sensitive to grass pollens with seasonal rhinitis and mild asthma, underwent bronchial challenge with carbachol in the pre-seasonal, seasonal, and post-seasonal period. According to pollen count, we observed significant differences between pre-seasonal, seasonal, and post-seasonal values. Also basal FEV1 values varied in the same fashion. Seven of the ten children also participated in a previous similar study in 1984, at which time they did not demonstrate significant variations to bronchial challenge. We wish to emphasize the importance of pollen concentration in the atmosphere as well as the existence of a threshold value of grass pollen necessary for a significant modification in bronchial reactivity to occur.