OBJECTIVES: To evaluate the autonomic functions in patients with ankylosing spondylitis (AS) by means of clinical and electrophysiological tests, to compare the data with those of healthy individuals and to investigate the relationship with the disease activity. PATIENTS AND METHODS: 32 asymptomatic AS patients and 30 healthy controls were included in the study. Parasympathetic functions were evaluated clinically with heart rate variability (HRV) and electrophysiologically with R-R interval variation (RRIV). Sympathetic functions were evaluated clinically with diastolic blood pressure response to isometric exercise (DBP) and electrophysiologically with sympathetic skin response (SSR). Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) and Ankylosing Spondylitis Disease Activity Score -C (ASDAS-C) were used to estimate the disease activity. RESULTS: HRV and RRIV was significantly lower in patients with AS when compared to controls, and in patients with BASDAI greater or equal to 4 when compared to the patients with BASDAI less than 4. There was no difference between the AS and the control groups and between the groups with BASDAI greater or equal to 4 and BASDAI less than 4 for DBP. Although there was no difference for SSR between AS and the control groups, SSR latency was significantly longer and SSR amplitude was significantly smaller in the group with BASDAI greater or equal to 4 when compared to the group with BASDAI less than 4. CONCLUSIONS: Our results indicate a parasympathetic dysfunction in AS patients, however the sympathetic system seems to be affected when the disease activity is increased. Patients with AS even they are asymptomatic must be investigated for autonomic dysfunction.