21 patients with acute carbon-monoxide poisoning had repeated brain scans by computerised tomography (CT) between May, 1977 and June, 1979. On the basis of the initial CT findings, the 21 patients could be divided into two distinct groups. In 11, areas of low density were seen bilaterally in the globus pallidus. These areas were still present 3 months after initial CT. In all but one of these patients the outcome was poor. The remaining 10 had no abnormal findings on their initial brain CT scans; outcome was good in this group.