The effect of ethanol ingestion on the efficacy of nasal continuous positive airway pressure (nasal CPAP) as a treatment for the obstructive sleep apnea (OSA) syndrome was studied in ten obese male subjects undergoing this therapy. On the first night of polysomnography, the lowest level of CPAP that maintained airway patency was determined (critical level). On the second (control) night (C), subjects slept the entire night on this level of CPAP. On the third night (E), subjects ingested either 1.5 ml/kg (part A, N = 6) or 2.0 ml/kg (part B, N = 4) of 50 percent ethanol (100 proof vodka) over one half-hour starting 1 h before bedtime. A serum ethanol level was obtained at bedtime (part A: 63.7 +/- 17.3 mg/dl; part B: 108.6 +/- 20.6 mg/dl), and subjects were monitored on the critical level of CPAP. Comparison of nights C and E for parts A + B showed no difference in total sleep time (TST) or the amount of different sleep stages as an absolute time or a percentage of TST except that there was more stage 2 (as a percent of TST) on E nights. The apnea + hypopnea index and C and E nights did not differ and was quite low (3.6 +/- 3.7/h vs 1.9 +/- 2.7/h). Similarly, ethanol ingestion did not increase the number of desaturations to at or below 90 and 85 percent, or lower the mean arterial oxygen saturation in NREM or REM sleep. Analysis of parts A and B separately also showed no differences with respect to the apnea + hypopnea index or the number of desaturations on control and ethanol nights. We conclude that acute moderate ethanol ingestion does not decrease the efficacy of an optimum level of nasal CPAP.