Risk factors for severe wheezing bronchitis were studied in children aged 4 months to 4 years in need of hospitalization. The children included in the study consisted of all cases generated from a geographically defined population, the catchment area of St Göran's hospital in Stockholm. The incidence was 3/1000 children and year, during the two years of observation, with the highest rate in boys under the age of 18 months (4.7/1000). Symptoms of a preceding upper respiratory tract infection were reported in 90% of the cases, but a viral etiology could only be demonstrated with virus isolation in 26%. Respiratory syncytial virus was the most common finding in younger children. Rhinovirus was primarily seen in older children with a history of previous wheezing. Regardless of whether the cases had a positive or negative virus isolation they showed the same seasonal distribution. Furthermore, there was no difference in risk factors between children with a positive and negative virus isolation. Children older than 18 months with negative virus isolation had higher IgE levels than those with positive isolation, suggesting that atopy is of greater importance in this group.