The case histories of four patients with severe cerebral manifestations as a result of CO poisoning are reported. All patients displayed an acute mesencephalic syndrome in the initial stage. One case developed a fatal bulbar syndrome following a transitional phase. Autopsy showed cerebral oedema, in addition to necrotic areas in the basal ganglia and cerebellar haemorrhage. The other 3 patients developed an apallic syndrome with all the typical features and symptoms. Two of these patients died without showing any improvement, displaying not only necroses in the basal ganglia, but also diffuse destruction of the white matter and, in one case, even of the cortex. The third patient recovered. His recovery took a similar course to that of a case of traumatic apallic syndrome and he was subsequently able to take up normal activities. It is concluded from these four cases that secondary brain oedema precedes the development of the most serious form of CO poisoning and is of grave prognostic significance.