Epidemiologic investigations of ambient ozone (O3) effects on daily asthma status have not used personal O3 exposures and have often lacked well-characterized allergen exposures. To address this, we studied 12 asthmatic subjects aged 9 to 16 yr, who recorded daily asthma symptoms (functional levels 0 to 5) and as-needed inhaler use during September and October 1993 in San Diego, California, Outdoor aeroallergens, O3, and fine particle concentrations were measured at a central outdoor site, and personal 12-h daytime exposures to O3 were measured daily. Personal O3 differed greatly between subjects and was 27% of mean outdoor O3. In random-effects autoregression models controlling for weekend days and fungal spores, personal O3 was associated with asthma severity: for a 90th percentile increase in O3 (25 ppb), symptom scores increased by 25% (95% CI: 0 to 49%) and inhaler use increased by 26% (95% CI: 3 to 48%) over their averages. Outdoor 12-h O3, but not 1-h maximum O3, was associated with inhaler use (p < 0.03). Fungal spores were significantly associated with symptoms (scores increased by 0.1 to 0.3/1,000 spores/m3) and inhaler use (0.1 to 0.4 puffs/1,000 spores/m3) across speciated groups. Pollen and fine particles (low levels) were not associated with any outcomes. These findings illustrate that the epidemiologic importance of O3 and allergenic cofactors can be underestimated by failure to account for personal O3 and fungal exposures.