A variable natural history was found in a standardized follow-up study of 118 patients with subacute sclerosing panencephalitis (SSPE). Only 20% followed the sequence:behavior change, mental deterioration, periodic attacks, severe debility, and death within a year. Six stages were identified: 0, subtle psychointellectual symptoms; 1, obvious psychointellectual and neurological changes; 2, stereotyped attacks; 3, vegetative psychomotor condition; 4, improvement; 5, relapse. Illness began in stage 0 for 32%; 1, 36%; 2, 19%; 1 and 2 simultaneously, 13%. In early development, 77% were hyperactive, and 27% had psychointellectual difficulties one to five years prior to obvious illness. The average duration of each stage was as follows: stage 0, two years; 1, 2.5 months; 2, 7.5 months; 3, 5.5 months; 4, 3.5 years; 5, 1.5 years. Noteworthy improvements and plateaus occurred in more than half of the patients. In stage 4, improvement was long-term and substantial for 5% and modest for 18%. Survival followed an exponential curve with an average half-life of 1.8 years; 41% of the patients survived beyond two years. Half of the patients passed through neither stage 0 nor stage 4; their half-life was 0.7 years. The remainder had a half-life of 3.0 years.