SYNOPSISAutonomic impairment in migraine is wall documented. In order to evaluate the autonomic control in migraine, spectral analysis of heart rate fluctuations was performed on ten migraine patients, drug‐free and during the inter‐headache phase. They were compared to nine healthy controls and eight tension headache patients. A 24h Holter recording of ECG was performed for each subject. Every half hour, a short ECG sub‐trace was digitized and submitted to R wave detection, followed by computation of heart rate power spectrum.The spectral analysis of heart rate fluctuations disclosed significant differences between control subjects and patients with migraine. The migraine patients displayed markedly enhanced low frequency fluctuations (below 0.1 sec‐1), during day hours (p<0.01) and especially at night (p<0.0006). In the respiratory frequency band (between 0.2 and 0.4 sec‐1) no significant change was observed. Tension headache patients however, resembled the controls in that they did not display enhanced low frequency fluctuations.The enhancement at low frequencies fluctuations only, a frequency range known to be related to vasomotor control, suggests that the migraine patients are characterized by a clear sympathetic instability. This finding supports the hypothesis that migraine is of neural origin and is consistent with the observation of large variations in regional cerebral flow. Spectral analysis of heart rate fluctuations allows us to specify and quantitate this autonomic imbalance and may provide a useful tool for the evaluation of drug therapy in