[': Diagnostic criteria for bronchiolitis are variable.', ': To study how the risk factors for recurrent wheezing and asthma vary by different definitions of bronchiolitis.', ': Viral etiology and atopic characteristics were studied in 259 hospitalized wheezing children (median age, 14 months; range, 0-36 months). The data were analyzed according to age (<6, <12, <24 and <36 months) and whether they had a history or no history of a previous wheezing episode. Sixteen viruses were detected by conventional and molecular methods. Atopic characteristics included the presence of eczema, specific and total IgE responses, blood eosinophil count, and modified asthma predictive index.', ': Evidence of respiratory virus infection was found in 93% of the cases and allergic sensitization in 26% of the cases. Rhinovirus infections and atopic characteristics (sensitization, blood eosinophil count, and modified asthma predictive index) increased by age and were significantly more common in children with recurrent wheezing episodes than in first-time wheezers in age categories of <24 and <36 months (P < 0.05 for all).'] Are bronchiolitis : age and previous wheezing episodes linked to viral etiology and atopic characteristics?