Hypercapnia observed in patients with chronic respiratory failure may not be an ominous sign for prognosis when they are receiving long-term oxygen therapy (LTOT). In this study, we selected 4,552 patients with chronic obstructive pulmonary disease (COPD) and 3,028 with sequelae of pulmonary tuberculosis (TBsq) receiving LTOT from 1985 to 1993 throughout Japan and prospectively analyzed their prognoses. The hypercapnic patients (PaCO2 >= 45 mm Hg) had a better prognosis than the normocapnic patients (35 <= PaCO2 < 45 mm Hg) for TBsq, but no difference was found between the two groups with COPD. Furthermore, Cox's proportional hazards model revealed that in TBsq hypercapnia was an independent factor for favorable prognosis, and that the relative risk for mortality was 0.76 in patients with 45 <= PaCO2 < 55 mm Hg, 0.64 for those with 55 <= PaCO2 < 65 mm Hg, and 0. 49 for patients with PaCO2 >= 65 mm Hg against normocapnic patients. This favorable effect of hypercapnia in TBsq was particularly apparent in the patients without severe airway obstruction. Even a rise of 5 mm Hg or more in PaCO2 over the initial 6- to 18-mo follow-up period was not associated with poor prognosis in TBsq, although it was in COPD. From these findings, we conclude that hypercapnia should not be generally considered an ominous sign for prognosis in those patients who receive LTOT.