Moderate-to-large quantities of alcohol are known to aggravate severe obstructive sleep apnoea (OSA), however, the reported effects of moderate alcohol consumption upon mild-to-moderate OSA are inconsistent. Given the reported benefits of moderate alcohol consumption on cardiovascular mortality, recommendations regarding the management of patients with OSA are difficult to formulate. The aim of this study was to evaluate the effects of moderate alcohol on sleep and breathing in subjects with mild-to-moderate OSA.Twenty-one male volunteers, who snored habitually, underwent polysomnography with and without 0.5 g alcohol . kg body weight (BW) -1 consumed 90 min prior to sleep time, in random order. The mean blood alcohol concentration (BAC) following alcohol at the time of lights out was 0.07 g . dL -1 . The distribution amongst the various sleep stages was not significantly altered by alcohol. The mean apnoea/hypopnoea index rose from 7.11.9 to 9.72.1 events . h -1 (meanSEM, p=0.017); however, there was no significant change in the minimum arterial oxygen saturation measured by pulse oximetry Sp,O 2 , apnoea length or snoring intensity. Mean sleep cardiac frequency rose significantly from 53.91.4 to 59.91.9 beats . min -1 (p<0.001) and overnight urinary noradrenalin increased from 14.92.3 to 18.82.3 nmol . mmol creatinine -1 (p=0.061) on the alcohol night compared to the nonalcohol night.To conclude, modest alcohol consumption, giving a mean blood alcohol concentration of 0.07 g . dL