BACKGROUND: Exposure to airborne fungi has been associated with increased airway hyperreactivity and asthma prevalence. OBJECTIVE: To investigate the association between common indoor fungi and airway hyperreactivity measured by peak expiratory flow variability in asthmatic children. METHODS: Children 6 to 12 years of age (n = 225) with a physician diagnosis of asthma were enrolled in the study to have their peak expiratory flow recorded twice daily during a 2-week period. Genus-specific, quantitative, in-home airborne mold concentrations were measured. Logistic regression models were used to examine the relationship between a mean peak expiratory flow variability greater than 18.5% (75th percentile) and any mold in the home (total mold, Cladosporium, Penicillium, Aspergillus, and Alternaria). RESULTS: Mold was detected in 93% of the homes. The most common molds were Cladosporium in 72% and Penicillium in 42% of the samples. Controlling for sex, ethnicity, age, and winter season of sampling, Penicillium measured in the home was associated with a mean peak expiratory flow variability greater than 18.5% (odds ratio, 2.4; 95% confidence interval, 1.2-4.8). Greater peak expiratory flow variability was not associated with total mold or other mold measured in the home. CONCLUSION: Exposure to airborne Penicillium is associated with increased peak expiratory flow variability in asthmatic children.