This study attempted to determine the relationship between average daily attendance (ADA) and costs in 72 Illinois high schools having only one attendance center. Costs included administrative, instructional, and operational costs. The schools were divided into three populations, namely all those under 5,000 ADA, those less than 2,000 ADA, and those less than 1,000 ADA. Data were obtained from records of the Illinois Office of Education, and both linear and curvilinear regression were employed. Regarding administrative costs, findings show that for all three populations there is economy of scale (lower costs associated with large enrollments), although there is some evidence of increasing costs beyond an optimal point. Instructional costs rise in schools below 1,000 ADA. Regarding operational costs, only the population of small schools showed economies of scale. Findings suggest that for at least smaller high schools there are economies of scale when size increases, although some of the findings are ambiguous. Results indicate that economy can be achieved by eliminating very small high schools. (Author/JM)