OBJECTIVE To verify whether hypoxemia affects respiratory muscle strength in the absence of malnutrition and whether such effect, if present, is reversible with an acute oxygen supplementation. METHODS Case series analysis, before-after trial. METHODS Outpatient pneumology departments of two university hospitals. METHODS One hundred twenty patients affected by COPD in stable conditions having actual to ideal body weight ratio of > or =90%. RESULTS Maximal inspiratory pressure (MIP) was measured at functional residual capacity level in the whole sample of subjects and during oxygen supplementation in 58 patients having a PaO2< or =60 mm Hg when breathing in room air. Predictors of MIP were assessed by a multivariate analysis. MIP values before and after oxygen supplementation were compared by a paired t test. MIP was independently correlated with FVC (p<0.001), PaO2 (p<0.01), and age (p<0.01). In the subgroup of hypoxemic patients, MIP values did not change significantly after oxygen supplementation (3.08+/-1.74 vs 3.03+/-1.91 kPa, t=0.43, not significant). CONCLUSIONS Hypoxemia is an important negative correlate of MIP even in well-nourished COPD patients. Its effect is not reversible with an acute oxygen supplementation.