Patients with interstitial lung disease (ILD) characteristically exhibit an increased ventilation and breathing frequency when awake. We wanted to see if these increases persisted during deep non-REM sleep. Using noninvasive techniques, we have quantified the pattern of breathing, arterial oxygen saturation, and transcutaneous PCO2 (PtcCO2) during standardized conditions of relaxed wakefulness and during Stage 4 (S4) sleep in eight patients with ILD and eight age-matched normal control subjects. The patients were given supplemental oxygen in order to prevent hypoxic ventilatory stimulation. The data were compared between the two groups during each of these states and also between states within each group. During wakefulness in the patients, respiratory frequency (f) and PtcCO2 were higher (p less than 0.001 and p less than 0.05, respectively) and inspiratory time (TI) and expiratory time (TE) were shorter (p less than 0.05 and p less than 0.001, respectively) than in the normal subjects. However, during S4 sleep, there were no significant differences between groups. Comparing wakefulness with S4 sleep: in the normal subjects during sleep, f and PtcCO2 were increased (p less than 0.01 and p less than 0.05, respectively), TE was shortened (p less than 0.01), and ventilation (VI) was unchanged. In contrast, in the patients, f decreased (p less than 0.001), TE lengthened (p less than 0.01), VI decreased (p less than 0.05), and the rise in PtcCO2 seen in the normal subjects during sleep did not occur.(ABSTRACT TRUNCATED AT 250 WORDS)