The aim of the present study was to compare hydrogen peroxide (H2O2) levels resulting from oxidative stress in exhaled breath condensate (EBC) and bronchoalveolar lavage (BAL) supernatants of subjects with different diffuse interstitial lung diseases (DILDs). Twenty-one patients who underwent BAL procedure for various DILDs were studied. EBC, which was collected the day before bronchoscopy, was obtained by cooling exhaled breath; BAL was processed for differential cellular count and supernatant was stored. H2O2 in both fluids was measured using a commercial fluorimetric kit with a limit of detection of 0.01 microM. No difference in H2O2 levels in EBC and in BAL was observed [median (range), 0.07 microM (0.01-0.6) and 0.08 microM (0.01-0.8), respectively]. No correlation was observed between BAL and EBC levels. H2O2 in BAL did not correlate with differential cellular count, whereas H2O2 EBC did correlate positively with the percentage of epithelial cells (r = 0.5, p = 0.007) and negatively with the number of macrophages (r = -0.4, p = 0.03). No correlation was observed between H2O2 levels either in EBC or BAL and lung function data. The data show that in DILDs H2O2 can be detected in both EBC and supernatants of BAL and that their relative concentrations are similar but not correlated with each other. The positive correlation between H2O2 levels in EBC and percentage of epithelial cells leads to the speculation that airway epithelia may play a relevant contribution in H2O2 production in the airway lumen.