Previous studies showed that treatment of obstructive sleep apnea syndrome (OSAS) with continuous positive airway pressure (CPAP) significantly reduced the blood pressure (BP) in hypertensive patients. We investigated the predictors of BP change in normotensive patients with OSAS who underwent CPAP. A total of 24 patients with OSAS (19 male; age: 48.7 +/- 10.4 years) were enrolled. The 24-hour mean BP (24 hMBP), subjective sleepiness, fasting venous blood samples, and anthropometric measurements were assessed at baseline, 6th week and 12th week of CPAP treatment. The 24 hMBP fell at 12 weeks from 89.2 +/- 8.4 to 82.9 +/- 7.3 mm Hg (P < .0001) irrespective of the severity of disease. Also, both daytime and nighttime BP showed significant reduction after CPAP. Male gender, Epworth sleepiness scale, body mass index, smoking, alcohol use, and baseline 24 hMPB were the independent predictors of a fall in 24 hMBP. The CPAP therapy may provide benefit even in the absence of overt hypertension by reducing both daytime and nighttime BP.