Twenty-five patients with allergic bronchopulmonary aspergillosis (ABPA) were observed for periods of 12 months to 10 years (average duration, 2.6 years) after initial therapy with prednisone, which was then tapered and discontinued unless maintained at minimal doses as required for control of asthma. Thirteen patients have had no recurrence, 4 patients did not comply with the initial regimen and could not be considered to be controlled, and 8 patients had 12 recurrent episodes of ABPA characterized by pulmonary infiltrates with no explanation other than ABPA. The exacerbations were closely correlated with sharp increases in total serum IgE, which subsequently decreased after resumption of prednisone therapy. The increase of IgE preceded the pulmonary infiltrates in 7 or 12 exacerbations. The exacerbations, characterized by increased serum IgE and pulmonary infiltrates, may be associated with minimal symptoms. Acute asthma without pulmonary infiltrates was not associated with increased IgE. Four exacerbations occurred during administration of beclomethasone diproprionate used for control of asthma, and therefore, this agent does not appear to prevent or reverse exacerbations of ABPA. Twelve exacerbations occurred in 8 persons, with 2 patients having 4 and 2 recurrences, respectively. This suggests that exacerbations are more likely to occur in certain patients. Serial measurements of total serum IgE appears to be a useful index of disease activity in ABPA. In the 4 patients who did not comply with the prednisone therapy regimen or regular physician visits, patterns of IgE changes, clinical evaluations, and chest roentgenograms were not of use in evaluation of the clinical state or progress of the patient. A treatment regimen is suggested for initial therapy and recurrences of ABPA on the basis of these observations.