We studied the immediate effects of continuous positive airway pressure (CPAP) applied nasally on the pattern of sleep in 12 patients, aged 30-58 years, with obstructive sleep apnea syndrome. All patients demonstrated a moderate to severe syndrome on the control night; apnea index ranged 28-83 apneas/h sleep. Nasal CPAP completely abolished all obstructive apneas and allowed apnea-free breathing in all 12 patients. Nasal CPAP had a marked effect on the sleep pattern. It significantly reduced stage I/II non-rapid eye movement (NREM) sleep and markedly increased stage III/IV NREM and REM sleep on the first treatment night. Stage I/II NREM sleep decreased from a control of 62.7 +/- 2.3% to 29.1 +/- 2.3% on the first treatment night. Stage III/IV NREM sleep increased from a control of 6.7 +/- 1.6% to 31.5 +/- 1.6%. The rebound in this sleep stage was especially marked in 3 patients aged 55-58 years. REM sleep increased from a control of 18.4 +/- 2.0% to 30.6 +/- 2.0% on the first treatment night. There was an increase in REM density. All patients were treated for another 2 nights and their sleep pattern analyzed on the third night. All sleep stages were still significantly different to the control night. The possible mechanisms involved are discussed.