BACKGROUND--Allergic bronchopulmonary aspergillosis is a serious complication of cystic fibrosis and may be difficult to diagnose. The aim of this study was to define the usefulness of measuring total IgE compared with other major criteria in the diagnosis of allergic bronchopulmonary aspergillosis in children with cystic fibrosis. METHODS--A retrospective analysis was carried out of the case records of 160 children attending a tertiary referral paediatric cystic fibrosis clinic. RESULTS--Sixteen children had a total IgE level above 500 IU/ml. Eleven children had six or more other major criteria and were considered to have allergic bronchopulmonary aspergillosis. These 11 children had a fourfold rise in IgE in association with clinical deterioration. A further child had a fourfold rise in IgE to 341 IU/l, and was also thought to have allergic bronchopulmonary aspergillosis. Eleven had a fall in IgE with successful treatment; one patient died with uncontrolled disease. Only one of these 12 children had negative precipitins to Aspergillus fumigatus. The five children with a raised IgE not thought to have bronchopulmonary aspergillosis had four or fewer major criteria and were not treated; none had positive precipitins. CONCLUSIONS--A fourfold rise in total IgE, particularly to above 500 IU/ml, is strongly suggestive of the diagnosis of allergic bronchopulmonary aspergillosis in children with cystic fibrosis. The measurement of total IgE has the merit of being simple to perform and objective. Positive aspergillus precipitins provide useful confirmatory evidence. These two criteria, taken in conjunction with clinical deterioration and new radiological shadowing, allow simplification of the diagnosis of allergic bronchopulmonary aspergillosis in cystic fibrosis.