Background: The aim of this study is to investigate the changes in the patterns of power spectra of R-R interval variability of diabetic patients who are subject to autonomic neuropathy. Methods: The changes in power spectra of eight diabetic patients were compared with those of eight normal persons while changing positions from supine to Trendelenburg and from supine to standing each. Results: Low, high and total frequency power densities of diabetic patient group were significantly lower than those of control group in resting supine position. Low frequency power density and ratio of low frequency power to high frequency power of control group increased significantly while changing position from supine to standing compared with those of diabetic patient group. And low frequency power density of control group decreased significantly compared with that of diabetic patient group while changing position from supine to Trendelenburg. Conclusions: The decrease in power densities of low, high and total frequency suggest depressed overall autonomic activities in diabetic patient group and significantly decreased changes in low frequency power and ratio of low frequency power to high frequency power while changing positions from supine to standing suggest attenuated sympathetic activity in diabetic patient group, reflecting autonomic changes in diabetic group rapidly.