To estimate the economic burden imposed by eastern encephalitis (EE), we identified a series of residents of eastern Massachusetts who had survived EE infection and enumerated any costs that could be attributed to their experience. The records of three people who suffered only a transient episode of disease were analyzed as well as those of three who suffered severe residual sequelae. Transiently affected subjects mainly required assistance for direct medical services; the average total cost per case was $21,000. Those who suffered persistent sequelae remained at home and seemed likely to live a normal span of years, but without gainful employment. Early in the course of their chronic illness, costs ranged as high as $0.4 million per year, but plateaued at about $0.1 million after three years. Hospital costs, which dominated early in the disease experience, approached $0.3 million per patient. Educational costs tended to replace hospital costs after two years as the dominant economic burden and totaled about $0.3 million per patient during the first six years. Total costs then averaged almost $0.8 million. By the time that these subjects will have reached 22 years of age, disease-related costs will have totaled about $1.5 million. Institutionalization will impose an additional lifetime cost of $1.0 million. Insecticidal interventions designed to avert outbreaks of human EE infection cost between $0.7 million and $1.4 million, depending on the extent of the treated region. The direct costs of an intervention are less than the $3 million imposed on one person suffering residual sequelae of EE.