To investigate the determinants of maximal expiratory flow (MEF) with aging, 17 younger (7 men and 10 women, 39 ± 4 yr, mean ± SD) and 19 older (11 men and 8 women, 69 ± 3 yr) subjects with normal pulmonary function were studied. For further comparison, we also studied 10 middle-aged men with normal lung function (54 ± 6 yr) and 15 middle-aged men (54 ± 7 yr) with mild chronic airflow limitation (CAL; i.e., forced expiratory volume in 1 s/forced vital capacity = 63 ± 8%). MEF, static lung elastic recoil pressure (Pst), and the minimal pressure for maximal flow (Pcrit) were determined in a pressure-compensated, volume-displacement body plethysmograph. Values were compared at 60, 70, and 80% of total lung capacity. In the older subjects, decreases in MEF (P < 0.01) and Pcrit (P < 0.05), compared with the younger subjects, were explained mainly by loss of Pst (P < 0.05). In the CAL subjects, MEF and Pcrit were lower (P < 0.05) than in the older subjects, but Pst was similar. Thus decreases in MEF and Pcrit were greater than could be explained by the loss of Pst and appeared to be related to increased upstream resistance. These data indicate that the loss of lung recoil explains the decrease in MEF with aging subjects, but not in the mild CAL patients that we studied.