The aim of the present study was to use the diaphragm electromyogram (EMGdi) to compare levels of neural respiratory drive (NRD) in a cohort of healthy subjects and chronic obstructive pulmonary disease (COPD) patients, and to investigate the relationship between NRD and pulmonary function in COPD. EMGdi was recorded at rest and normalised to peak EMGdi recorded during maximum inspiratory manoeuvres (EMGdi % max) in 100 healthy subjects and 30 patients with COPD, using a multipair oesophageal electrode. EMGdi was normalised to the amplitude of the diaphragm compound muscle action potential (CMAPdi,MS) in 64 healthy subjects. The mean¡SD EMGdi % max was 9.0¡3.4% in healthy subjects and 27.9¡9.9% in COPD patients, and correlated with percentage predicted forced expiratory volume in one second, vital capacity and inspiratory capacity in patients. EMGdi % max was higher in healthy subjects aged 51–80 yrs than in those aged 18–50 yrs (11.4¡3.4 versus 8.2¡2.9%, respectively). Observations in the healthy group were similar when peak EMGdi or CMAPdi,MS were used to normalise EMGdi. Levels of neural respiratory drive were higher in chronic obstructive pulmonary disease patients than healthy subjects, and related to disease severity. Diaphragm compound muscle action potential could be used to normalise diaphragm electromyogram if volitional inspiratory manoeuvres could not be performed, allowing translation of the technique to critically ill and ventilated patients. KEYWORDS: Chronic obstructive pulmonary disease, electromyography, respiratory diaphragm O bjective markers of disease severity that reflect the physiological load on the respiratory system in chronic obstructive pulmonary disease (COPD) are currently lacking. Although COPD severity is categorised in terms of forced expiratory volume in one second (FEV1) in management guidelines [1], correlations between FEV1 and breathlessness [2] or quality of life are modest [3], and reported relationships between FEV1 and prognosis are inconsistent [4– 6]. Two small studies confirm that neural respiratory drive (NRD) is increased in COPD [7] and relates to symptoms [8], but the use of measurements of NRD to assess disease severity in COPD has not been fully investigated, in part because there are no data to define ranges of NRD within the healthy population. In COPD, mechanical abnormalities including airflow obstruction, static and dynamic hyperin-flation and intrinsic positive end-expiratory pressure increase the load on the respiratory