A radioimmunoassay (RIA) has been developed for neurone-specific enolase (NSE) and used to measure serum levels in patients with a range of neurological disorders. Serum NSE levels were within the normal range in 21 patients with multiple sclerosis and 4 patients with Guillain-Barre syndrome. Normal serum NSE levels were also recorded in patients with motor neurone disease, anterior spinal thrombosis, multi-infarct disease, benign intracranial hypertension and peripheral neuropathy. However, two patients in coma, one as a result of encephalitis, the other due to subarachnoid haemorrhage (SAH) had elevated serum NSE. In the former, serum NSE levels appeared to predict a deterioration in clinical state, levels later returning to normal before an improvement in clinical condition, In the patient with SAH, levels were elevated on admission and remained elevated until death. Serum NSE levels may be of use in predicting outcome in patients with acute neurological disease.