In COPD, dynamic hyperinflation (DH) occurs during exercise and during metronome-paced tachypnea (MPT). We investigated the relationship of DH with breathing pattern and ventilation (V˙E) in COPD and normal subjects (NS).In 35 subjects with moderate COPD and 17 younger healthy volunteers we measured inspiratory capacity (IC), breathing frequency (f R ), expiratory time (T E ), ventilation (V˙E) and end-tidal carbon dioxide tension (P ET CO 2 ) at baseline and after 30s of MPT at 40breaths/min with metronome-defined I:E ratios of 1:1 and 1:2. A reduction in IC (ΔIC) was taken to indicate DH. In COPD subjects, DH correlated with T E but not with V˙E or P ET CO 2 , and was best predicted by total lung capacity. NS also showed DH (although less than in COPD), which correlated with P ET CO 2 but not with f R , T E or V˙E.We conclude that MPT evokes DH in both NS and patients with COPD. T E is the most important determinant of DH during MPT in patients with COPD.