Introduction. The manifestations of autonomic nervous system (ANS) dysfunction in autoimmune diseases have been the subject of many studies. However, the published results pertaining to such research are controversial. Sudden cardiac death due to fatal arrhythmias is frequent in patients with systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA). Objective. To analyze risk predictors of sudden cardiac death related to the degree of autonomic dysfunction. Methods. We performed cardiovascular ANS assessment in 90 patients in this case-controlled study, including 52 (6 male, 46 female) patients with SLE, 38 (6 male, 32 female) with RA and 41 (23 male, 17 female) healthy subjects. The methodology included a comprehensive ECG analysis (with Schiller software AT-10) of QTc interval, late potentials, short-term heart rate variability (HRV) and nonlinear HRV (Poincare plot) analysis; 24-hour Holter ECG monitoring with ECG QTc interval analysis, HRV analysis; 24-hour blood pressure monitoring with systolic and diastolic blood pressure variability; cardiovascular autonomic reflex tests (according to Ewing). Vagal dysfunction was established by performing 3 tests: Valsalva maneuver, deep breathing test and heart rate response to standing test. Dysfunction of the sympathetic nervous system was examined by applying 2 tests: blood pressure response to standing and handgrip test. Results. In all cardiovascular reflex tests, the frequencies of abnormal results were significantly higher among the patients than among the healthy subjects. Severe autonomic dysfunction was more common in RA. QTc interval was more prolonged in patients with SLE. Both diseases were associated with depressed heart rate variability compared to controls, the reduction being greater in RA patients. In the patients with SLE, autonomic dysfunction is predominantly with higher sympathetic activity while in RA vagal predominance is evident. Conclusion. SLE and RA are associated with severe autonomic dysfunction and the presence of significant risk predictors related to the onset of sudden cardiac death.