Epidemiological studies have repeatedly demonstrated the effects of acidic particles on lung function. Three consecutive panels of children participating in a summer camp in the Austrian Alps were investigated. On-site pollution assessment consisted of 24-h measurements of particulate matter < 10 microns and the hydrogen, sulfate, and ammonium ion components. The 24-h maximums of ozone and daily pollen counts were also established. For 47, 45, and 41 subjects, daily FEV1, FVC, and peak expiratory flow were recorded. Fifteen percent, 11%, and 5% of participants, respectively, reported current asthma medication, indicating a markedly increased prevalence of respiratory disease. Mean levels of ambient pollutants were approximately 15% higher for the first panel than for the other two panels, but the hydrogen ion (H+) component was twice as high for Panel 1. The maximum H+ exposure during Panel 1 was 84 nmol/m3 (4 micrograms/m3 H2SO4 equivalent). For FEV1 in Panel 1, a significant decrease of -0.99 ml per nmol/m3 H+ (p = 0.01) was observed. For Panel 2, the FEV1/H+ coefficient was found to be similar (-0.74 ml per nmol/m3 H+; p = 0.28), while for Panel 3 it was in the opposite direction (0.10 ml per nmol/m3 H+; p = 0.83). The decrease in FEV1 observed in Panel 1 was more pronounced when the mean exposure during the previous 4 d was considered (-2.99 ml FEV1 per nmol/m3 H+; p = 0.004). We conclude that summer haze acidic particles may be associated with transient decreases in lung function in children.