Forty-one patients with mumps encephalitis examined at the Department of Paediatrics, University of Helsinki, during the period Jan. 1, 1968, to Dec. 31, 1980, were reviewed with special reference to clinical appearance and outcome. The ratio of males to females was 4:1 and the age range 1.2 to 13.7 years. The clinical findings were high fever greater than or equal to 39 degrees C in 83.0%), impairment of locomotion and balance (36.6%), seizures (24.4%), psychic disorders (22.0%), depressed level of consciousness (19.5%), vertigo (12.9%) and/or gastric pains (12.9%). One patient with congenital toxoplasma and cytomegalovirus infections died. On leaving the hospital eight were still ataxic, one of these was not able to speak and two had difficulties in concentration. One patient was transferred to another hospital because of psychosis. At the follow-up examination 4 to 24 months after the onset of the disease two patients were ataxic and seven suffered from behavioural disturbances. Electroencephalography (12.2% of all and 25% of examined patients) showed generalized slow wave disturbance at follow-up in six patients and borderline disturbances in eight.