Carbon monoxide (CO) poisoning is the most common form of lethal poisoning. The aim of this prospective clinical study was to assess the possible role of S100B, the structural protein in the astroglia, as a biochemical marker of brain injury in carbon monoxide poisoning. Serum S100B determination was performed in 38 consecutive patients poisoned by carbon monoxide who were admitted to the Emergency Department (ED) in Ljubljana. All three unconscious patients had elevated S100B levels. The patient with the highest S100B died. S100B was elevated in two of the six patients with initial transitory unconsciousness at the scene. All 29 patients without loss of consciousness had normal S100B levels. Carbon monoxide poisoning appears to be associated with elevated S100B levels.