Study objective: To classify abnormal lung function in epidemiologic studies, we first calculated 'normal' values using data from Australian white adults. We then examined the effects of airway hyperresponsiveness (AHR), respiratory symptoms, current and past asthma, and current smoking on spirometric function. Methods: A large random sample of 1,527 adults aged 18 to 73 years was studied. We measured respiratory symptoms and smoking history by questionnaire and AHR by histamine inhalation test. Results: Data from 729 'normal' subjects (asymptomatic nonsmokers without AHR) were used to obtain regression models for FVC, FEV1, peak expiratory flow rate, and forced expiratory flow between 25% and 75% of FVC. The R2 values were 0.76, 0.74, 0.58, and 0.20, respectively. The presence of AHR reduced FVC by 0.1 L and FEV1 by 0.2 L, on average. Subjects with asthma-related symptoms had a mean reduction in FVC of 0.1 L for both genders and in FEV1 of 0.08 L for women and 0.2 L for men. Current asthma reduced FVC by 0.3 L, on average, and FEV1 by 0.5 L for women and 0.6 L for men. The FEV1 was reduced by 0.002 L per cigarette smoked daily. Conclusion: Recent symptoms, AHR, and current smoking were all important predictors of reduced lung function.