Flows measured from maximal expiratory flow-volume (MEFV) curves were used to evaluate the efficacy of postural drainage in improving ventilatory function acutely. Maximal expiratory flow-volume curves were obtained for 9 cystic fibrosis subjects and 10 subjects with chronic bronchitis before and 5, 15 and 45 min after a 30 min session of postural drainage with percussion, vibration and coughing. Forced vital capacity (FVC) was significantly increased 45 min after drainage for the combined group. Flows at high lung volumes were different for the 2 subgroups. Subjects with cystic fibrosis demonstrated a significant increase in peak expiratory flow rates 45 min after drainage and an increase in forced expiratory volume in 1 s at all time intervals. The subjects with chronic bronchitis had a decreased peak expiratory flow rate 5 min after drainage, but by 45 min it had returned to baseline. There was no significant change in 1 s forced expiratory volume at any time interval for the chronic bronchitis subgroup. Changes in flows at low lung volumes were similar for the 2 subgroups. At 45 min after drainage there was an increase in flow rates near 50% of FVC. Flows near 25% of FVC were increased 15 and 45 min after drainage. Postural drainage with coughing apparently resulted in significant improvement in flows at low lung volumes. Changes in flows at high lung volumes were less consistent.