OBJECTIVE: Fibromyalgia (FM) is a prevalent and poorly understood disorder associated with a significant amount of disability. Some clinical characteristics are common to both FM and vasovagal syncope (which is caused by dysautonomia). We assessed the response of patients with FM to a head up tilt table test (HUT). We also examined sympathovagal balance by assessing heart rate variability (HRV). METHODS: We studied 17 women with FM and 14 female control subjects. After baseline functional assessments, they underwent a 3 stage HUT (with isoproterenol). HRV was assessed over a 24 h period and also before and during HUT. Quality of life was assessed using the Medical Outcomes Study SF-36 Short Form Health Survey. RESULTS: HUT was positive in 64.7% of the patients with FM compared with 21.3% of controls (p = 0.016). FM patients had less HRV, as measured by either time domain or frequency domain analysis. The FM group had a different response to HUT than controls. Quality of life was significantly lower in patients with FM compared to controls (p < or = 0.001 in all domains). CONCLUSION: Patients with FM have abnormal responses to 2 tests of autonomic nervous system function. Further research is needed to determine if dysautonomia plays a role in the pathogenesis of FM or is a result of FM.