Acute bronchitis is a common syndrome characterized by upper respiratory tract infection accompanied by productive cough in persons without chronic lung disease. As a first step in investigating the potential role of pulmonary airflow disturbances in acute bronchitis, 27 adult patients with acute bronchitis performed serial pulmonary function tests and daily peak flow measurements and completed daily symptom diaries. Eleven patients (40 percent) had a forced expiratory volume of the first second (FEV1) at entry less than 80 percent of predicted. Mean FEV1, forced midexpiratory flow rate (FEF25-75), and peak flow measures showed steady improvement over the five-week period. Differences between initial and final flow rates were statistically significant. These changes were particularly evident when two groups were created by stratifying by "abnormal" (FEV1 less than or equal to 80 percent predicted) and "normal" (FEV1 greater than 80 percent predicted) initial FEV1. Total duration of cough and subjective ratings of cough severity were not predicted by initial FEV1, but work absence was significantly higher in the abnormal group. The finding of reversible airway obstruction suggests a role for bronchospasm in many cases of acute bronchitis and calls for further research regarding proof of rapid reversibility and treatment with bronchodilators.