OBJECTIVE Hypoxemia is known to be associated with abnormal heart rate variability (HRV) that can reflect the severity of the illness and may have prognostic value in patients with chronic obstructive pulmonary disease (COPD). The aim of this study was to examine the relationship between the derangements in cardiac autonomic nervous function and the oxygenation status or degree of airflow obstruction in COPD patients by using HRV analysis. METHODS Thirty clinically stable COPD patients and 18 age-matched normal subjects were included in this study. The normalized high-frequency power (nHFP) and the low-/high-frequency power ratio (LFP/HFP) were used as indices of vagal activity and sympathovagal balance, respectively. RESULTS Although global HRV measures were all significantly decreased, the nHFP and LFP/HFP of COPD patients were not significantly different from those of normal controls. There was a negative correlation between nHFP and arterial partial pressure of O2 (PaO2) and a positive correlation between LFP/HFP and PaO2 in COPD patients. No correlation existed between forced expiratory volume in 1.0 s/forced vital capacity (FEV1/FVC), % predicted of FEV1 (%FEV1) and nHFP or LFP/HFP in COPD patients. CONCLUSIONS The resting autonomic nervous function of COPD patients is not different from that of normal controls. Though the degree of airway narrowing is not related to the cardiac autonomic nervous function, chronic hypoxemia can lead to enhanced cardiac vagal activity and depressed sympathetic activity in COPD patients. A worse oxygenation status is associated with increased cardiac vagal and decreased cardiac sympathetic activities in COPD patients.