Fifty-one infants hospitalized because of acute bronchiolitis and 24 control children were prospectively followed until 2 years of age. Respiratory syncytial virus was detected initially in 31 index children, parainfluenza virus in two, and rhinovirus in one. More index children were born during the months April through September, and six index children had neonatal respiratory complications. The index children were breast-fed for shorter periods, lived in more crowded homes, and had more siblings. No significant differences were found for atopic background and atopic illness at 2 years of age. The index children had more respiratory infections and episodes of bronchopulmonary obstruction during the follow-up period (60% with three or more episodes). To characterize the hospitalized children, a discriminant analysis was performed. A discriminant function classified 47 of 51 index children as belonging to the index group and 21 of 24 control children to the control group.