Sleep apnea syndrome (SAS) is an important risk factor for hypertension and cardiovascular diseases. Diurnal blood pressure (BP) changes are evaluated by 24 h ambulatory blood pressure monitoring (ABPM). The purpose of this study was to clarify the relationship between diurnal BP variation and SAS severity, as well as the impact of antihypertensive therapy on diurnal BP variation. Patients seen at our clinic between April and September 2006 with excessive daytime sleepiness or apnea were enrolled. All patients had polysomnography and ABPM. Mean 24 h BP and nighttime BPs were significantly higher in the SAS group than in the non-SAS group. No significant differences were observed in daytime BPs between the two groups. SAS patients had a high mean 24-h BP and an elevated nighttime BP, both of which increased as SAS severity increased. Nighttime BPs were significantly higher in the moderate SAS group than in the non-SAS group. Nighttime BP and morning BP were significantly higher in the severe SAS group than in the non-SAS group. With respect to antihypertensive agents' effects on diurnal BP changes, there were no significant differences between the SAS and non-SAS groups. In conclusion, compared with non-SAS patients, patients with SAS had a higher 24-h BP, especially nighttime BP. Patients with moderate SAS tended to have elevated nighttime BP. In patients with severe SAS, elevated BP was sustained during the night despite the use of antihypertensive agents.