To analyze the function of cardiac autonomic regulation in patients with obstructive sleep apnea syndrome (OSAS), we enrolled 36 patients with OSAS and divided them according to the apnea hypopnea index (AHI) into 2 groups: Group I (n=19) had mild OSAS (AHI < 20) and Group II (n=17) had severe OSAS (AHI > or = 20). The findings were compared with those of 24 healthy control subjects who were matched for age, sex, blood pressure, and body mass index. All participants underwent 24-hour Holter monitoring, with continuous time-dependent and spectral analysis of heart rate variability. In addition, we performed arrhythmia analysis. Frequent or repetitive ventricular arrhythmias (> or = 30 premature ventricular beats/hour) were detected in 15 (42%) patients with OSAS and in 6 (25%) members of the control group. In both mild and severe OSAS, SDNN was significantly lower than in controls, and SDANN findings were similar. In mild OSAS, RMSSD values were not significantly lower than in controls, but in severe OSAS they were. The ULF, VLF, LF and LF/HF values of both groups of OSAS patients were significantly higher than those of controls, but their HF values were lower. The mean LF/HF ratio during the same period was significantly higher in Group II than in Group I and the control group. Our results suggest that cardiac autonomic activity may be altered in patients with OSAS throughout a 24-hour period, that this alteration occurs even in the absence of hypertension, heart failure, or other disease states, and that it is linked to the severity of OSAS.