It was hypothesized that obstructive sleep apnea may precipitate myocardial ischemia, reflected by ST-segment depression, in some patients during sleep. Overnight sleep studies and simultaneous 3-channel Holter monitoring were performed on 23 consecutive patients with obstructive sleep apnea without a history of coronary artery disease. Each patient was randomly assigned to nasal continuous positive airway pressure for the first half of the night. An episode of significant ST depression was defined as > 1 mm from baseline for > 1 minute. The total duration (minutes) of ST depression was indexed to the total sleep time (minutes per hour of sleep). Seven patients (30%) had ST depression during sleep. In all 7 patients the duration of ST depression decreased during nasal continuous positive airway pressure (30 +/- 18 vs 11 +/- 13 minutes per hour of sleep) in association with a reduction in the apnea-hypopnea index (65 +/- 35 vs 7 +/- 6/hour), arousal index (49 +/- 14 vs 6 +/- 4/hour) and the duration that oxygen saturation was < 90% (44 +/- 27 vs 12 +/- 23% total sleep time). When patients were not on nasal continuous positive airway pressure, the apnea-hypopnea and arousal indexes were higher during periods of ST depression than when ST segments were isoelectric, whereas oxygen saturation was not different. These 7 patients underwent exercise testing, which was positive for inducible myocardial ischemia in 1 patient. It is concluded that ST depression is relatively common in patients with obstructive apnea during sleep and that the duration of ST depression is significantly reduced by nasal continuous positive airway pressure.(ABSTRACT TRUNCATED AT 250 WORDS)