The pattern of change in arterial oxyhemoglobin saturation (SaO2%) during sleep was characterized in 13 patients with interstitial lung disease (ILD), 12 of whom had restrictive ventilatory impairment. Four patients snored during sleep. During the studies, 9 patients had unequivocal rapid eye movement (REM) sleep episodes. The total duration of each patient's REM episodes averaged 49 min (range, 26 to 93 min), which was 22 +/- 7% (1SD) of the total sleep duration. Seven of these 9 patients were nonsnorers but had definite falls in SaO2% during REM sleep (mean fall in SaO2%, 8 +/- 3%), and in 6 of them the falls in SaO2% were transient, with a mean duration of 28 +/- 12 s and a total duration of 6.4 +/- 3.9 min or 16 +/- 12% of the total REM sleep duration. The other nonsnorer showed sustained desaturation (SaO2, 80 to 85%) for his entire REM sleep period of 26 min. In the nonsnoring patients, the falls in SaO2% during REM sleep (8 +/- 3%) were usually greater than those occurring during awake exercise (6 +/- 7%). Two snorers had unexpected sleep apnea syndrome (minimal SaO2% during NREM sleep, 83 and 77%, respectively; minimal SaO2% during REM sleep, 58 and 67%, respectively). The other snorers had greater than 10% falls in SaO2% during NREM sleep. The breathing frequency in NREM sleep in patients with ILD (mean, 23 +/- 5 breaths/min) was persistently above the normal range (mean, 15 +/- 0.4 breaths/min). The possibility of sleep hypoxemia should be considered in the management of patients with ILD.