BACKGROUND--The relation between plasma fluoride levels and bronchial responsiveness was investigated in a longitudinal study in aluminium potroom workers who reported work-related asthmatic symptoms. METHODS--From a cross-sectional respiratory survey, 26 men who reported work-related asthmatic symptoms on a validated questionnaire were selected for repeated measurements of bronchial responsiveness to methacholine. Regular analyses of plasma fluoride levels were performed. Exposure to fluoride and total particulates was assessed from routine surveillance of the workplace. Bronchial responsiveness was expressed as the dose-response slope of the line through the origin and last data point. RESULTS--A positive association was found between bronchial responsiveness and plasma fluoride levels, such that an increase in the plasma fluoride level of 10 ng/ml was associated with an increase in the dose-response slope by a factor of 1.11 (95% confidence interval 1.05 to 1.17). Plasma fluoride levels were associated with the total atmospheric fluoride concentration in mg/m3 (beta = 28.1), but not with total particulates in the environment. CONCLUSIONS--Bronchial responsiveness in aluminium potroom workers reporting work-related asthmatic symptoms appears to be related to plasma levels of fluoride. The underlying mechanism is, however, unknown.