Allergic bronchopulmonary aspergillosis (ABPA) is an unusual syndrome caused by hypersensitivity to Aspergillus spores growing in the bronchii. Previous investigators have suggested that the IgE levels and precipitating antibodies may vary according to disease activity. We have been able to closely follow 12 out of a group of 40 ABPA patients with IgE and serum precipitating antibody measurements. Our results confirm that both the total serum IgE and the precipitin response vary according to ABPA disease activity. In particular the IgE trend appears to mirror the disease activity in that a rising level may portend a flare, while a stable or declining value implies disease remission.