BACKGROUND: The etiology of cardiological syndrome X (CSX) is unknown, however the dysfunction of cardiac autonomic nerves has been reported. AIM: The aim of the study was to evaluate changes in autonomic nervous system (ANS) activity in CSX patients. METHODS: Forty-three patients (age 48 +/- 9 yrs) with CSX and forty-three healthy volunteers (age 47 +/- 9 yrs) as a control group were included in the study. The function of ANS was assessed using non-invasive methods: heart rate variability (HRV) and tests based on cardiovascular reflexes. RESULTS: According to HRV analysis the patients were divided into two groups: A--22 pts with decreased HRV parameters and B--18 pts without significant differences as compared with the controls. At rest there was a difference in the HRV parameters between CSX patients and the controls. The CSX patients had the advantage of sympathetic component (the higher ratio LF/HF and LF value). Additionally an increase in heart rate and rate-pressure product was observed in 58% of CSX patients. In CSX patients the 30/15 ratio was decreased in response to changes in the body position in about 45% of CSX patients. A deep breathing test revealed a drop in the Expiration to Inspiration ratio. CONCLUSIONS: In patients with cardiological syndrome X selected tests of ANS indicated the presence of disturbances in the sympathetic-parasympathetic balance. In 40% of CSX patients the response to all autonomic tests was diminished. This indicates that the population of CSX patients is heterogeneous with normal and low abnormal ANS activity.