We have compared the symptomatic benefit of air and oxygen at rest in hypoxemic patients with chronic obstructive airway disease (COAD) or interstitial lung disease (ILD). A total of 12 severely disabled patients with COAD (mean +/- SEM, PaO2, 50.3 +/- 3.7 mm Hg) and 10 with ILD (PaO2, 48.0 +/- 3.1 mm Hg) received 28% oxygen and air by Venturi face mask, each gas on two occasions, in a double-blind randomized fashion. SaO2 increased (p less than 0.01) in both groups during oxygen breathing: COAD, 85.1 +/- 2.3% versus 93.1 +/- 1.4%; ILD, 85.5 +/- 1.7% versus 94.7 +/- 0.9%. The patients with COAD stated that air helped their breathing on 15 of 24 occasions and that oxygen helped on 22 of 24 occasions (p less than 0.05). In the patients with ILD the values were 6 of 20 and 13 of 20 occasions, respectively (p less than 0.05). In both groups of patients the severity of breathlessness recorded on a 100-mm visual analog scale was significantly (p less than 0.05) lower during oxygen breathing: COAD, 29.6 +/- 4.5 versus 45.6 +/- 6.0; ILD, 30.2 +/- 5.1 versus 48.1 +/- 4.4. Ventilation measured by magnetometers was significantly lower during oxygen breathing in the patients with COAD (8.2 +/- 1.0 versus 9.3 +/- 1.1 L/min; p less than 0.05), but the difference between oxygen and air in patients with ILD was not statistically significant (9.3 +/- 1.3 versus 11.2 +/- 1.6 L/min; p greater than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)