Acute bronchitis is a common reason for visits to primary care physicians and a commonly given reason for antibiotic treatment. However, evidence regarding the efficacy of antibiotics for this syndrome is lacking. In a randomized trial, a one-week course of a frequently used antibiotic, doxycycline, was compared with one week of placebo in 74 otherwise healthy adults with acute bronchitis. The doxycycline group fared no better than the placebo group for all 13 outcomes measured, including duration of cough, clinical improvement at one week, return visits for unresolved symptoms, days away from work, and subjective ratings of cough severity, sleep loss, diminished activity and overall well-being. Doxycycline is not beneficial in the treatment of acute bronchitis in otherwise healthy adults.