OBJECTIVES: To assess the interaction between the sympathetic and parasympathetic systems in patients with fibromyalgia syndrome (FM), using power spectrum analysis (PSA) of heart rate variability (HRV). In addition, we explored the association between HRV, measures of tenderness, FM symptoms, physical function, psychological well being and quality of life. METHODS: We studied 22 women with FM and 22 age-matched healthy women. Twenty-minute electrocardiogram recordings were obtained in a supine position during complete rest. Spectral analysis of R-R intervals was done by the fast-Fourier transform algorithm. RESULTS: Heart rate was significantly higher in FM patients compared with controls (P < .006). FM patients had significantly lower HRV compared with controls (P= .001), and higher low-frequency (LF) and lower high-frequency (HF) components of PSA than controls (P < .001). Quality of life, physical function, anxiety, depression, and perceived stress were moderately to highly correlated with LF, HF (in normalized units), and LF/HF. No association was observed between HRV parameters and measures of tenderness and FM symptoms. CONCLUSIONS: The basal autonomic state of patients with FM is characterized by increased sympathetic and decreased parasympathetic tones. Autonomic dysregulation may have implications regarding the symptomatology, physical and psychological aspects of health status.