The objective of the study was to define the impairment of sympathovagal balance in patients with diabetes mellitus (DM) and coronary heart disease (CHD) compared to healthy controls (HC) showing similar heart rate variability (HRV) at supine rest. 88 DM (41 m, 47 f; age 62 +/- 1 years; BMI 27.1 +/- 1.5 kg/m2; HbA1c 7.9 +/- 0.4%), 49 CHD (27 m, 22 f; age 62 +/- 1 years; BMI 27.1 +/- 1.6 kg/m2; HbA1c 5.2 +/- 0.1%) and 16 HC (8 m, 8 f; age 59 +/- 1 years; BMI 26.4 +/- 0.5 kg/m2; HbA1c 5.0 +/- 0.1%) were investigated. Time series of heart period duration (HPD) were obtained during 2 min deep breathing (6/min), 5 min of supine rest and for 5 min at upright position using a RR memory device (BHL 6000, Baumann-Haldi Switzerland, modified ECG lead, 1 kHz). Mean HPD, coefficient of variation (CV), total power (TP) and integral power in the HF (0.15 to 0.5 Hz), MF (0.05 to 0.15 Hz) and LF (0.015 to 0.05 Hz) frequency bands as well as (MF-HF)/(MF + HF) as spectral index were calculated. As to be expected we found significantly lower values of CV, TP and HPD in DM compared to HC. The CV of HRV did not differ significantly between DM and CHD but TP and HPD of CHD patients were significantly higher in comparison to DM. Therefore, the deterioration of HRV was most pronounced in the DM group. For further analysis we calculated data of subjects with CV\'s in the upper quartile (> or = 3.52) of the CV at supine rest. The aim of this procedure was to compare subjects with similar high HRV at supine rest. With this method we obtained from all subjects 12 HC, 11 DM and 12 CHD. These DM had a significant decrease of CV, TP and the integral power at the HF frequency band during active orthostasis compared to HC and CHD. The spectral index increased significantly during standing in HC and CHD but was unchanged in DM. These changes were accompanied by a nearly similar increase of HRV during deep breathing. In conclusion, DM with normal reaction to deep breathing did not activate the sympathetic baroreflex loop during active orthostasis. This could be an early sign of sympathetic dysfunction in DM with normal HRV at supine rest.