Maximal mid-expiratory flow (MMEF), maximal expiratory flow volume (MEFV) curves obtained with a wedge spirometer, and nitrogen closing volumes were determined in 38 patients with mild airway obstruction. Seventeen patients had asthma in remission and 21 had bronchitis. In all of them the forced expiratory volume in one second was within the normal range. Results were compared with predicted data in the literature and with a group of normal control subjects. In the patients with asthma, compared to predicted flow rates, MMEF was abnormal in 5, MEFV curves were abnormal in at least 8; closing volume was abnormally increased in only one patient, and an abnormal slope of the alveolar plateau was present in 4 additional patients. In the patients with bronchitis, compared to predicted flow rates, -MEF was reduced in 5, MEFV curves were abnormal in at least 7; increased closing volumes were present in 6, and the slope of the alveolar plateau was abnormal in 3 other patients. When flow rates were compared with those of normal control subjects, MMEF was about as frequently abnormal as MEFV curves, suggesting that the discrepancy between abnormal MMEF AND MEFV curves was due to variability of the predicted data. The results indicated that flow rates can be abnormal in subjects with normal closing volumes and a normal slope of the alveolar plateau, and that MEFV curves can be more sensitive than closing volume in detecting abnormalities in patients with mild airway obstruction. The results suggested that the use of both MEFV curves and the closing volume test for screening would defect functional abnormalities more frequently than either test alone.