Facial temperature and amplitude of capillary pulsations from the forehead and cheeks were measured shortly before and after pharmacological blockade of the stellate ganglion in 9 patients with reflex sympathetic dystrophy of an upper limb, and in 1 other patient with erythromelalgia of all four limbs. Patients were then heated to determine the effect of sympathetic blockade on mediation of thermoregulatory facial flushing Release of vasoconstrictor tone following stellate ganglion blockade was invariably followed by an increase in orbital and cheek temperature, and by similar but less consistent increases in temperature of the forehead, lips and chin. The amplitude of capillary pulsations recorded from the forehead and cheeks also increased on the side of stellate ganglion blockade. Flushing of the forehead and cheek on the sympathetically intact side during body heating far outweighed the extent of flushing after release of vasoconstrictor tone. The thermoregulatory response was prevented by sympathetic blockade, indicating that an active sympathetic vasodilator pathway had been interrupted. In contrast, the temperature of the orbit during body heating was not influenced by sympathetic blockade. Asymmetry of forehead temperature was detected before sympathetic blockade in 8 of 9 patients with reflex sympathetic dystrophy, suggesting that autonomic disturbances in this condition may influence cervical sympathetic outflow. The results indicate that sympathetic vasodilator fibres passing through the stellate ganglion mediate thermoregulatory facial flushing, and that release of vasoconstrictor tone has only a minor influence on this response in most areas of the face.