A record of fifty-three patients admitted to the Infectious Diseases Department of the Royal Free Hospital between April 1955 and September 1957 suffering from 'epidemic neuromyasthenia' establishes the fact that the condition was endemic in the general population before, during and after the outbreak among the staff of the hospital. A further outbreak occurred in North Finchley between 1964 and 1967 and sporadic new cases are still being encountered. The majority of these patients show evidence of involvement of the central and sympathetic nervous systems and the reticulo-endothelial system. Abnormal muscular fatigability is the dominant clinical feature and it is suggested that mitochondrial damage may provide an explanation for this phenomenon. Enzyme tests carried out in seven cases show pathologically high levels of lactic dehydrogenase, and glutamic oxalo-acetic transaminase. A follow-up study suggests that there is one group of patients that recovers completely or nearly completely, a second that recovers but is subject to relapses and a third that shows little or no recovery, these patients remaining incapacitated.