Background: The technique of stellate ganglion block (SGB) is used widely in the treatment of painful disease, but its mechanism still remains unknown. Objective: To explore the carotid hemodynamic features of patients with severe headache and its correlation with the effect of SGB. Design: A quasi-randomized controlled, single-blind and longitudinal observational trial was conducted. Setting and participants: This trial was completed in Tiantan Hospital of Capital University of Medical Sciences. The subjects were 30 outpatients (15 males and 15 females) with severe headache in Tiantan Hospital between February and August, 1998. Methods: Thirty patients with severe headache were selected as the trial group, and 30 healthy people as the control group. The hemodynamic parameters of bilateral carotid and vertebral arteries were measured by Color Doppler ultrasound ASONICS, SPEC-TRA before SGB and 5 minutes after SGB. The clinical degrees of pain in the trial group before and after SGB were assessed by 5-point verbal rating scale (VRS-5). Main outcome measures: Blood flow, resistance index (RI), peak value velocity (PKV), time average velocity (TAV), diameter of blood vessels in both groups and pain scores of the trial group were measured before and after SGB. Results: Miosis at the side of block was observed in the trial group 1 minute after SGB. Before SGB, PKV (66.14 ± 18.2), RI (0.74 ± 0.05) and VRS-5 (4.5 ± 0.5) of the pain-affected sides were significantly higher in the trial group than in the control group (46.20 ± 7.21, P < 0.01; 0.65 ± 0.05, P < 0.05; 0, P < 0.05); after SGB, however, these parameters in the trial group decreased to 59.47 ± 9.21 (P < 0.05), 0.65 ± 0.05 (P < 0.05) and 1.2 ± 0.6 (P < 0.01), respectively. Conclusion: SGB is effective in the treatment of severe headache. Abnormal cerebrovascular tension was found in the patients with severe headache, so SGB may dually regulate cerebrovascular tension in the dominated location by balancing vegetative nerve, and thus to relieve headache.