192 patients, most of them ambulatory, with acute exacerbation of chronic bronchitis were treated with 10- to 14-day courses of oral ofloxacin receiving 400 mg once daily or doxycycline first 200 mg and then 100 mg once daily in this randomized observer-blinded multicentre study. 98 patients were included in the ofloxacin group and 94 in the doxycycline group. The clinical efficacy was 86/87 (98.8%) in the ofloxacin group and 87/91 (91.2%) in the doxycycline group. The bacteriological efficacy was 31/34 (91.2%) in the ofloxacin group and 19/24 (79.2%) in the doxycycline group. Some patients had more than one pathogen. There was no statistically significant difference in efficacy between the two treatment groups; both treatments were highly effective and well tolerated. The adverse events showed an expected pattern. Ofloxacin treatment was discontinued in three patients. In conclusion, both treatments were found to have similar safety and efficacy and were well tolerated when used for treatment of acute exacerbations of chronic bronchitis.