Daytime blood pressure (BP) in 31 male patients with obstructive sleep apnea syndrome (OSAS) was measured and the effects of nasal continuous positive airway pressure (CPAP) treatment on daytime BP were studied. Subjects were 48 +/- 10 (mean +/- SD) years old and weighed 80 +/- 13 kg. The mean systolic BP and diastolic BP were 135 +/- 15 mmHg and 88 +/- 14 mmHg, respectively and daytime hypertension was present in 12 (38%) subjects. Apnea index (AI) and the lowest oxygen saturation during sleep were significantly more severe in the hypertensive (HT) than in the non-hypertensive (NHT) patients (p < 0.05). AI was significantly correlated with diastolic BP (p < 0.05) and the mean and lowest oxygen saturation during sleep were significantly correlated with both systolic (p < 0.05) and diastolic BP (p < 0.01). After nasal CPAP treatment for two weeks, both systolic and diastolic BP were significantly reduced; the former from 135 +/- 15 mmHg to 126 +/- 10 mmHg (p < 0.005) and the latter from 88 +/- 14 mmHg to 78 +/- 6 mmHg (p < 0.001). These data form direct evidence that daytime hypertension is partially induced by OSAS and is reversible with nasal CPAP treatment.