The effect of ambient air pollutants, pollens, and mold spores on respiratory health was studied in an area with low concentrations of chemical pollutants and abundant aeroallergens. A panel of 40 asthmatic subjects living near East Moline, Illinois, recorded peak expiratory flow rates (PEFRs), respiratory symptoms, frequency of asthma attacks, and asthma medication use between April and October 1994. Daily outdoor concentrations of pollutants and aeroallergens were measured, and indoor levels of bioaerosols were measured on several occasions in each participant's home. Ozone was associated with increased morning and evening symptom scores and decreased evening PEFR, and these associations remained significant with adjustment for weather and aeroallergens. The association between ozone and asthma medication use was increased in magnitude and significance with adjustment for weather and aeroallergens; however, the association between ozone and morning PEFR became nonsignificant with weather and aeroallergen adjustment. Significant associations were also found between pollen concentration and decreased evening PEFR, as well as between increased morning and evening symptom scores and asthma medication use. In addition, associations were noted between total spore concentration and increased morning PEFR and decreased morning and evening symptom scores. The inverse associations found with mold spore concentrations were not consistent with the results of other studies; however, the associations between ozone and pollen concentration were consistent with previous studies. When results were stratified by a number of independent risk factors, no differences were noted relative to allergic status or presence of dampness or flooding in the home; however, the associations with outdoor ozone and pollens were seen mainly among participants with low levels of exposure to indoor bioaerosols (< 1,800 spores/m3) or with no environmental tobacco smoke exposure.