Symptomatic exacerbations are frequent problems in the management of chronic bronchitis and bronchial asthma. Identification of a bacterial etiology as the cause of specific exacerbations should be based on changes in clinical symptoms and documentation of significant bronchial bacterial flora and a neutrophilic inflammatory response. Most acute bacterial exacerbations in patients with bronchitis or asthma are caused by Hemophilus influenzae, Streptococcus pneumoniae, or Branhamella catarrhalis. Treatment with ampicillins, synthetic tetracyclines, or trimethoprim/sulfamethoxazole is successful in 80 to 90 percent of bacterial exacerbations. Emergence of resistant Hemophilus species and pneumococci motivates development of new orally administered antimicrobial drugs. Appropriate treatment depends on the prompt recognition that bacterial infection is present. Once instituted, antimicrobial therapy should be continued for a minimum of 10 to 14 days, which should increase the duration of the infection-free period until the next bacterial exacerbation. Adequate response should be evaluated by the return of symptoms to pre-infectious levels and by decreased sputum bacterial flora and neutrophilic inflammation.