The validity of peak inspiratory mouth pressure (P.PI-max) as a measure of inspiratory muscle strength was investigated by comparing it with sniff Pes in patients with COPD with respect to (1) learning effect, (2) reproducibility, and (3) measures of agreement. To assess the discriminating capacity of P.PImax, we compared the values in patients with COPD with those of healthy elderly subjects. Thirty-four patients (mean age, 62.5 years) with severe airways obstruction (FEV1, 44% predicted; FEV1/IVC, 37% predicted) and 149 healthy subjects (age > or = 55 years) were included. P.PImax was assessed during a maximal static inspiratory maneuver, while sniff Pes was assessed during a maximal sniff maneuver. Both maneuvers were performed from residual volume ten times on the same day. P.PImax showed no learning effect, while the sniff maneuver used seven attempts to obtain a maximal value. The intraindividual coefficients of variation of P.PImax and sniff Pes were 11.2% and 6.0%, respectively. Measures of agreement showed no significant discrepancies between the mean P.PImax and mean sniff Pes (0.29 kPa, p = 0.49). There was a significant correlation (r = 0.57, p < 0.001) between both measurements. P.PImax was significantly (p < 0.001) lower in both male (8.2 kPa) and female (6.2 kPa) patients with COPD compared with healthy men (11.0 kPa) and healthy women (8.8 kPa). We conclude that P.PImax is a valid and noninvasive assessment of inspiratory muscle strength.