Simultaneous bronchial biopsy and serum samples were obtained from 14 patients after a single oral dose of 200 mg ofloxacin and from ten patients during the course of multiple dose oral treatment, (200 mg ofloxacin bd) for acute exacerbations of chronic bronchitis. Duplicate bronchial samples from different parts of the lung were obtained from five patients. Concentrations in bronchial mucosa were equal to or higher than serum concentrations. There were no statistically significant differences in concentrations after single or multiple doses but there appeared to be a trend towards higher concentrations after multiple doses. After a single dose bronchial and serum concentrations ranged from 1.3 to 15.5 mg/kg and from 0.1 to 5.0 mg/l, respectively, between 1 and 6 h after dosing. After multiple dosing of patients with acute exacerbations of chronic bronchitis bronchial and serum concentrations ranged from 1.7 to 21.0 mg/kg and 1.0 to 6.1 mg/l, respectively between 1 and 12 h. The percentage difference in ofloxacin concentrations in five duplicate bronchial samples from different parts of the lung was -4%, (95% confidence intervals +22% to -30%). Thus, the model of bronchial sampling after single dosing of patients before diagnostic bronchoscopy gave results that were similar to those obtained after multiple dosing of patients with acute exacerbations of chronic bronchitis. After multiple dosing ofloxacin concentrations were greater than 1 mg/l or greater than 1 mg/kg in serum and bronchial mucosa throughout the dosing interval, which is above the MIC for most respiratory pathogens.