In a double-blind study of 137 patients with exacerbation of chronic bronchitis and chronic obstructive lung disease, the efficacy and safety of ofloxacin was compared with that of trimethoprim-sulfamethoxazole (TMP/SMX). Both groups improved. The frequency of severe adverse reactions was highest in the TMP/SMX group, and 14.9% of the patients discontinued the treatment. In the ofloxacin group 6% had to stop the treatment. The failure rate was significantly lower in the ofloxacin-treated patients, 3.2% versus 13.8% in the TMP/SMX group. Ofloxacin was found to be an effective drug with few adverse reactions.