Eighty children with wheezy bronchitis were followed prospectively for 12 years. At the end of the follow-up period only 22 (28%) still had symptoms of asthma. Forty-three children (54%) had ceased to wheeze before the age of 3 years, four children between 3 and 7 years of age and 11 children between 7 and 11 years of age. Of the 22 children who still had asthma, all but one were much improved, although 70% of them noticed asthmatic symptoms during exercise. Heredity for asthma/wheezing, allergy, the occurrence of eczema, and onset of wheezing after 18 months of age were associated with an increased risk of persistent asthma. Allergy had developed in 59% of the children with persistent asthma and in 10% of those who had stopped wheezing. Serum IgE was above the mean +1 SD in 45% and above the mean +2 SD in 24% of the children at the end of the 12-year follow-up. A serum IgE above the mean +2 SD was found in 8 of 13 children with asthma combined with proven allergy, but only in 1 of 9 children with asthma without allergy. Surprisingly, 8 of 48 children who had stopped wheezing and had no clinical allergy had as high IgE levels as the children with asthma and allergy, which reduced the allergy predictive value of a high serum IgE to 36%. Some of these high IgE levels seemed to be a family trait.