Allergic bronchopulmonary aspergillosis, a hitherto uncommon but potentially crippling complication of asthma, occurs at an unknown prevalence in the United States. Using both modern serologic criteria complete with a history of asthma and radiologic findings, we were able to make the diagnosis of allergic bronchopulmonary aspergillosis in 28 of 100 consecutive patients with asthma who had immediate cutaneous reactivity to Aspergillus fumigatus and who were seen in an outpatient setting. Problems associated with use of less comprehensive criteria for this diagnosis are discussed.