Pulmonary functions of patients with allergic bronchopulmonary aspergillosis were studied during an acute episode (n = 6); during a mean follow-up period of 44 months (range four months--14.8 years) (n = 16); and for any correlation between duration of ABPA and asthma with the total lung capacity (helium dilution), 1 second forced expiratory volume (FEV1), vital capacity, 1 second forced expiratory volume-forced vital capacity ratio (FEV1:FVC per cent) and diffusing capacity of carbon monoxide (DL:CO) (single breath) for the entire group (n = 22). All patients were treated with corticosteroids (intermittent or continuous) and bronchodilators. For the 16 patients, slopes using linear regression analysis were determined from the function as per cent predicted versus time in months from diagnosis and then analyzed for significance. Significant functional loss was shown in three of 16 patients for FEV1, two of 16 patients for vital capacity, one of 16 patients for FEV1:FVC per cent, none of 10 patients for DL:CO and one of 10 patients for total lung cital capacity, FEV1:FVC per cent and the duration of asthma or allergic bronchopulmonary aspergillosis was found by multiple regression analysis correcting for age and smoking (mean 4.24 years; range 0.3 to 14.8 years). Roentgenographic criteria and blood eosinophilia were used to define a "flare" of allergic bronchopulmonary aspergillosis. The six patients during a flare showed a significant reduction in total lung capacity (P less than 0.001), vital capacity (P less than 0.05), FEV1 (P less than 0.01) and DL:CO (P less than 0.001) which uniformly returned to baseline values during steroid therapy. The FEV1:FVC per cent remained unaltered. These findings, contrary to suggestions in the literature, indicate that in the majority of our patients there was no significant progressive functional deterioration after diagnosis. However, during acute episodes of allergic bronchopulmonary aspergillosis, transient reduction of volumes and DL:CO were uniformly present.