Increasingly, exhaled breath condensate (EBC) is being used to sample airway fluid from the lower respiratory tract. EBC pH may be a biomarker of airway inflammation in chronic obstructive pulmonary disease (COPD). In this study, the reproducibility of EBC pH in COPD was investigated. A total of 36 COPD patients and 12 healthy nonsmoking subjects participated in several investigations: duration of argon deaeration, within-sample variability, effect of freezing, leaving samples at room temperature, nose-peg use, within- (WD) and between-day (BD) variability. Analysis of repeated measurements was performed using the Bland-Altman method with limits of agreement (LOA; mean difference+/-2 SD). Wider LOA indicate greater variability. EBC pH became significantly higher with argon deaeration for < or =5 min. Variability during sample analysis was minimal; LOA of within-sample variability, freezing for 3 months and leaving at room temperature for 3 h were -0.29-0.45, -0.37-0.42 and -0.13-0.09, respectively. In contrast, variability due to nose-peg use (LOA -1.46-1.99), WD (LOA -1.50-2.48) and BD variability (LOA -2.52-3.02) were higher in COPD. In healthy nonsmoking subjects, nose-peg use (LOA -0.27-0.23), WD (LOA -0.33-0.40) and BD variability (LOA -0.46-0.44) were more reproducible. In conclusion, the variability of exhaled breath condensate pH in chronic obstructive pulmonary disease patients is mainly due to changes in airway pH over time, which are not seen in healthy nonsmoking subjects. Reasons for these fluctuations in exhaled breath condensate pH are unclear and require further investigation.