This article reviews the existing literature about acute bronchitis, a condition commonly diagnosed but poorly defined. The little epidemiologic research that has been done has failed to identify a microbiologic etiology approximately 60% to 85% of the time. The majority of cases appear to be caused by viruses, but 25% of adults with nonspecific lower respiratory symptoms may actually have pertussis.Mycoplasma pneumoniaeand Chlamydia pneumoniae probably play minor roles. Although clinicians frequently prescribe antibiotics to patients they have diagnosed with acute bronchitis, there is little evidence in support. General treatment studies have failed to demonstrate benefit, and the natural history of even potentially curable pathogens is not altered by antimicrobial therapy. Some recent studies suggest that albuterol may be the best treatment choice for acute bronchitis; it can successfully ameliorate symptoms, and does not pose the same public health risk as inappropriate antibiotics do. Erythromycin may occasionally be indicated for patients in frequent contact with small infants not yet immunized against pertussis, but careful surveillance of the child is probably more effective than treating the contagious adult.