Much about acute bronchitis in children is uncertain, even its definition. This study was undertaken to help build a clinical definition to serve as a guide for further research. Charts of 180 children 1 to 12 years old were reviewed. Each had received a diagnosis of acute bronchitis (54 patients), upper respiratory infection with a cough (128 patients), or a new diagnosis of asthma (26 patients) during the previous 12 months. A history of sputum production (odds ration 25.0, p = 0.0001), the finding of rales or rhonchi on examination (OR 11.6, p less than 0.0001), and a past history of lower respiratory illness (OR 3.0, p = 0.01) were associated with a diagnosis of acute bronchitis, but fever and duration of illness were not. In addition, the diagnosis of acute bronchitis was strongly associated with a prescription for an antibiotic. In a child with a cough, acute bronchitis is more likely to be diagnosed if there is a history of sputum production, rales or rhonchi on examination, and a past history of lower respiratory disease.