Twenty-one patients in whom the diagnosis of subacute necrotizing encephalomyelopathy (SNE, Leigh's disease) was made, have been treated with large doses of thiamine hydrochloride, thiamine propyldisulfide (TPD) or thiamine tetrafurfuryldisulfide (TTFD). In 12, the diagnosis is secure and in 9 it is presumptive. The incidence and quality of remissions in patients in both categories exceeded those described in the literature in 84 untreated patients with this disease. In general, remissions correlated well with high thiamine levels in the cerebrospinal fluid and exacerbations with the reverse. The ultimate failure of vitamin therapy in several patients who initially did well seemed to correlate with the tendency of CSF thiamine levels to fall during the course of continuous therapy. Preliminary results indicate that substituted thiamine derivatives (TPD and TTFD) were more efficacious in raising CSF thiamine levels than thiamine. TTFD has the further advantage of imparting little odor to treated patients. In the one case in which a comparative study was done, the intravenous route was more effective than the oral one in raising blood and CSF levels but in high dosage therapy raised CSF levels satisfactorily at least in the early days and weeks of treatment of many cases. While thiamine and its derivatives appear to be of benefit, a permanent solution to the therapeutic problem will probably come from another source.