As part of a multicenter clinical trial (Lung Health Study), methacholine inhalation challenge testing was performed in 5,877 current cigarette smokers, ages 35 to 59 yr (mean 48.5 +/- 6.8 yr), with borderline to moderate airflow limitation (FEV1/FVC ratio 63.0 +/- 5.5). The test was successfully completed in 96.4% of subjects, of whom 63% were male and 95.9% were white. Symptomatic reactions to methacholine were rarely severe enough to require evaluation by a trial physician. Nonspecific airways hyperresponsiveness (AHR) was defined as a greater than or equal to 20% decline in FEV1 from the post-diluent control value after inhalation of less than or equal to 25 mg/ml methacholine. AHR was noted in a significantly higher percentage of women (85.1%) than men (58.9%). Moreover, nearly twice as many women as men (46.6 and 23.9%, respectively) responded to less than or equal to 5 mg/ml of methacholine. In both men and women, baseline degree of airways obstruction and clinical center were strongly associated with AHR (p less than 0.001), whereas age was not. Additional associations with AHR were analyzed in men and women separately using logistic regression after adjustment for baseline lung function, age, and center-to-center differences. In men, AHR was significantly related to symptoms of wheeze, chronic cough and/or sputum, and a history of asthma or hay fever (p less than 0.004), but not to current or lifetime tobacco use. By contrast, among women, AHR was not significantly associated with chronic cough and/or phlegm (p greater than 0.05) or a past history of asthma or hay fever (p greater than 0.1) and was only weakly related to wheeze and current asthma (p = 0.04), as well as to cigarette pack-years (p = 0.044). These results indicate that most continuing smokers with functional evidence of early chronic obstructive pulmonary disease have nonspecific AHR that is strongly related to gender and baseline lung function and, to a lesser extent, to respiratory symptoms. The reason for the striking effect of gender on AHR in early chronic obstructive pulmonary disease is unclear but cannot be attributed to male-female differences in age, cigarette use, presence of asthma, or baseline degree of airflow obstruction.