Twenty-three patients with focal, strictly unilateral lesions of the peripheral nervous system were examined by infrared-thermography. The Minor sweat test was used to determine if sympathetic outflow was disturbed. In fifteen patients without a concomitant sympathetic lesion (controls) thermosymmetry was not disturbed. Eight patients had evidence of abnormal sweat secretion. In these patients thermoregulation was severely disturbed. During the first 5 to 8 months, affected skin areas were hyperthermic, whereas later only hypothermia was observed. Cold stimuli increased temperature differences in patients with disturbed sympathetic function, but not in controls. Thermography is a reliable, non-invasive technique to detect a lesion of sympathetic outflow and permits an estimation of the time-course of the lesion.