Bronchial responsiveness to methacholine was examined in a Norwegian general population sample (n = 490) 18 to 73 yr of age. Altogether, 20 and 6% of the sample had PC20 less than or equal to 32 mg/ml and PC20 less than or equal to 8 mg/ml, respectively. The relationship of bronchial responsiveness to the following potential predictors were examined: sex, age, smoking habits, airway caliber (FEV1), FEV1 percent predicted (%FEV1), urban-rural area of residence, occupational airborne exposure in present job, and total serum IgE. After adjusting for age and FEV1, the odds ratio for PC20 less than or equal to 32 mg/ml was higher for men than for women in smokers and in ex-smokers, but did not vary by sex in nonsmokers, the adjusted odds ratio for PC20 less than or equal to 32 mg/ml in male compared with female smokers being 8.4 (95% Cl: 2.5-37.4). Irrespective of smoking status the sex- and FEV1-adjusted odds ratio for PC20 less than or equal to 32 mg/ml fell with increasing age. For every 10-yr increase in age the adjusted odds ratio for PC20 less than or equal to 32 mg/ml methacholine in nonsmokers decreased by 2.0 (95% Cl: 1.3-3.3). Also FEV1 and %FEV1 were predictors of PC20 less than or equal to 32 mg/ml after adjusting for sex and age irrespective of smoking status. Bronchial responsiveness (PC20 less than or equal to 8 mg/ml) was more prevalent in rural than in urban areas, the adjusted odds ratio being 2.5 (95% Cl: 1.1-5.9) for bronchial responsiveness in rural compared with urban residents after adjusting for sex, age, smoking habits, and FEV1.(ABSTRACT TRUNCATED AT 250 WORDS)