This is a randomized, open-label, parallel group, multicenter study to determine the effectiveness and safety of 488 mg of levofloxacin (once daily by mouth for 5 - 7 days) compared with 250 mg of cefaclor (every 8 hours for 7 - 10 days) in adults with chronic bronchitis experiencing rapid onset of worsening of symptoms caused by bacterial infection. The study consists of 3 visits: one visit for screening and enrollment, and 2 visits for assessment of safety and effectiveness (one visit on Days 3 - 5 of the study and one visit [post-therapy] 5 - 7 days after the last dose of the study drug). The total duration of patient participation in the study is approximately 2 weeks. Levofloxacin and cefaclor are antibacterial agents used for the treatment of many types of infections, including infections with a rapid onset and brief duration caused by bacteria. The primary assessment of effectiveness in this study is the microbiologic response to treatment (the rate of elimination of disease-causing bacteria, by patient, and by type of bacteria), 5 - 7 days after the last dose of study drug. Safety evaluations (incidence of adverse events, physical examination, laboratory tests) are performed throughout the study. The study hypothesis is that treatment with levofloxacin is at least as effective and as well tolerated as treatment with cefaclor in adult patients with chronic bronchitis experiencing sudden worsening of symptoms caused by bacterial infection. Levofloxacin 488 mg by mouth once daily for 5 - 7 days, or cefaclor 250 mg by mouth every 8 hours for 7 - 10 days.