A descriptive study of acute bronchitis in patients without pre-existing pulmonary disease was undertaken in the community during the winter months of 1986-87. Forty-two episodes were investigated in 40 individuals. The cardinal symptom was the acute onset of cough (100%), usually productive (90%). Wheezing was noted by 62% of patients, but heard on auscultation in only 31%. A potential pathogen was isolated in 29% of cases with a virus (eight cases) being identified more frequently than either Mycoplasma pneumoniae (three cases) or a bacterium (three cases). The acute illness was associated with significant reductions in forced expired volume in 1 second (P less than 0.02) and peak expiratory flow (P less than 0.001) but not forced vital capacity compared to 6 weeks later. Ten of the 27 (37%) patients who had a histamine challenge test performed at 6 weeks had a PD20 of less than 7.8 mumol histamine. Thirty-nine episodes (93%) were treated with antibiotics by the general practitioner, the clinical course being unremarkable apart from one patient who developed a lingular pneumonia despite antibiotic therapy. Further studies are required to assess whether acute bronchitis causes an acute increase in bronchial hyperresponsiveness and whether either antibiotics or inhaled bronchodilators or anti-inflammatory therapy has a useful role in the management of this predominantly viral illness.