OBJECTIVES: The prevailing linear reductionist medical model seems unable to explain complex multisymptomatic illnesses such as fibromyalgia (FM) and similar maladies. Paradigms derived from the complexity theory may provide a coherent framework for these elusive illnesses. Along these lines is the proposal that FM represents a degradation of our main complex adaptive system (the autonomic nervous system, ANS), in a failed effort to adjust to a hostile environment. Healthy complex systems have fractal structures. Heart rate fractal-like variability reflects resilient ANS performance. Our aim was to measure the heart rate variability (HRV) fractal scaling index in FM patients and to correlate this index with clinical symptoms. METHOD: We studied 30 women with FM and 30 controls. All participants filled out questionnaires assessing the severity of FM. The HRV fractal scaling index was estimated during 24 h using detrended fluctuation analysis (DFA). RESULTS: The fractal scaling index alpha-1 was higher in FM patients than in controls (mean +/- sd: 1.22 +/- 0.10 vs. 1.16 +/- 0.09; p = 0.031). There was a positive correlation between the fractal scaling index alpha-1 and the visual analogue scale (VAS) for depression (Spearman\'s rho = 0.36, p = 0.04). CONCLUSIONS: The heart rate fractal exponent alpha-1 is altered in FM patients, suggesting a rigid ANS performance. This tangible non-linear finding supports the notion that FM may represent a degradation of our main complex adaptive system, namely the ANS.