Staging criteria for COPD (chronic obstructive pulmonary disease) include symptoms and lung function parameters, but the role of reduced inspiratory muscle strength related to disease severity remains unclear. Therefore the present study tested whether inspiratory muscle strength is reduced in COPD and is related to disease severity according to GOLD (Global Initiative for Chronic Obstructive Lung Disease) criteria and assessed its clinical impact. PImax (maximal inspiratory mouth occlusion pressure), SnPna (sniff nasal pressure) and TwPmo (twitch mouth pressure) following bilateral anterior magnetic phrenic nerve stimulation were assessed in 33 COPD patients (8 GOLD(0), 6 GOLD(I), 6 GOLD(II), 7 GOLD(III) and 6 GOLD(IV)) and in 28 matched controls. Furthermore, all participants performed a standardized 6 min walking test. In comparison with controls, PImax (11.6+/-2.5 compared with 7.3+/-3.0 kPa; P<0.001), SnPna (9.7+/-2.5 compared with 6.9+/-3.3 kPa; P<0.001) and TwPmo (1.6+/-0.6 compared with 0.8+/-0.4 kPa; P<0.001) were markedly lower in COPD patients. TwPmo decreased with increasing COPD stage. TwPmo was correlated with walking distance (r=0.75; P<0.001), dyspnoea (r=-0.61; P<0.001) and blood gas values following exercise (r>0.57; P<0.001). Inspiratory muscle strength, as reliably assessed by TwPmo, decreased with increasing severity of COPD and should be considered as an important factor in rating disease severity and to reflect burden in COPD.