OBJECTIVE: To evaluate the effects of orthostatic stress with a head-up tilt on the autonomic nervous system and to determine how a cerebral stroke influences the cardiac autonomic function, using the power spectral analysis(PSA) of heart rate variability(HRV). METHOD: We studied 11 stroke patients with a left hemiplegia and 14 patients with a right hemiplegia. Their hemispheric brain lesions were confirmed by the MRI. The ECG and respiration signals were recorded at the tilt angle of 0o and 70o for 5 minutes under the condition of frequency controlled respirtaion(0.25 Hz). Data were compared with the age- and sex-matched 12 healthy controls. RESULT: In a control group, the normalized high frequency power showed a significant decrease during the head-up tilt(p<0.05), whereas the normalized low frequency power showed a significant increase(p<0.05). But for the left and right hemiplegia groups, there were no significant changes of normalized high and low frequency power under the orthostatic stress(p>0.05). Compared with the right hemiplegia and control groups, the left hemiplegia group was associated more with a reduced low and high frequency power and showed no significant changes under the orthostatic stress. CONCLUSION: PSA of HRV can identify the reduced cardiac autonomic activity in stroke patients, with a greater reduction in the left hemiplegia group than in the right hemiplegia group, which may cause a high risk of cardiac arrhythmias and sudden death.