The aim of this study was to test the hypothesis that neural respiratory drive, measured using diaphragm electromyogram (EMGdi) activity expressed as a percentage of maximum (EMGdi%max), is closely related to breathlessness in chronic obstructive pulmonary disease. We also investigated whether neuroventilatory uncoupling contributes significantly to breathlessness intensity over an awareness of levels of neural respiratory drive alone. EMGdi and ventilation were measured continuously during incremental cycle and treadmill exercise in 12 chronic obstructive pulmonary disease patients (forced expiratory volume in 1 s±sd was 38.7±14.5 % pred). EMGdi was expressed both as EMGdi%max and relative to tidal volume expressed as a percentage of predicted vital capacity to quantify neuroventilatory uncoupling. EMGdi%max was closely related to Borg breathlessness in both cycle (r=0.98, p=0.0001) and treadmill exercise (r=0.94, p=0.005), this relationship being similar to that between neuroventilatory uncoupling and breathlessness (cycling r=0.94, p=0.005; treadmill r=0.91, p=0.01). The relationship between breathlessness and ventilation was poor when expansion of tidal volume became limited. In chronic obstructive pulmonary disease the intensity of exertional breathlessness is closely related to EMGdi%max. These data suggest that breathlessness in chronic obstructive pulmonary disease can be largely explained by an awareness of levels of neural respiratory drive, rather than the degree of neuroventilatory uncoupling. EMGdi%max could provide a useful physiological biomarker for breathlessness in chronic obstructive pulmonary disease.