A number of studies have shown that children who had infantile bronchiolitis are at increased risk of recurrent episodes of wheezing. A genetic predisposition to atopy is mentioned in some studies and is contested by others. Lung function abnormalities and increased bronchial responsiveness (BR) have been described after infantile bronchiolitis. We investigated children who had had the clinical syndrome of bronchiolitis during infancy and compared them with asthmatic and healthy children of the same age regarding bronchial caliber, smooth muscle tone, and responsiveness to histamine. Lung function was measured by forced oscillometry. We found that most children with current symptoms had either decreased baseline bronchial caliber, increased bronchial smooth muscle tone, or increased BR. These patients are comparable to mild asthmatics. The children without current symptoms are comparable to healthy children in these respects. Recurrent respiratory symptoms after bronchiolitis should be regarded as mild asthma and treated as such.