Increased nonspecific bronchial responsiveness (NSBR) may be a risk factor for the development of chronic airflow obstruction. We evaluated this hypothesis in a cohort of 378 underground coal miners and working nonminers. Methacholine testing was performed at the beginning and end of a 5-yr study period. Spirometry was repeated at 6-mo intervals and individual 5-yr FEV1 slopes were calculated by linear regression. Relationships between FEV1 slopes and NSBR were examined using multiple linear regression models, controlling for FEV1 level, smoking, and mining. Increasing NSBR at the initial survey was associated with a somewhat greater rate of subsequent FEV1 decline. Methacholine responders at the final survey had a considerably increased rate of decline during the previous years. Responsiveness status changed over the 5 yr in 22% of the subjects. Both the development and persistence of increased NSBR were strongly associated with higher rates of FEV1 decline. In contrast, FEV1 declines were not accelerated among workers with increased NSBR that reverted to normal. Smoking and mining were both independently associated with FEV1 declines, but did not substantially modify the effect of NSBR. Due to its variability over time, NSBR testing predicts lung function decline only in some individuals, and its value as a prognostic test for chronic airway disorders is limited. Because improvement in bronchial hyperresponsiveness was associated with a reduction in the rate of FEV1 loss, interventions directed at preventing or reducing nonspecific airway hyperresponsiveness should be investigated.