An analysis relating district size in terms of average daily attendance (ADA) to costs in terms of current operating expenditures reveals that districts operating most efficiently in 1974 are unit districts at 2,432 ADA, high school districts at 874 ADA, and elementary districts at 336 ADA. Each population is treated separately and, thus, the "optimal" sizes cannot be added together. It should be noted that these simple gross, or bivariate, functions have relatively low explanatory power. Loss of pupils will probably focus more attention on the high costs of small schools in Illinois. However, it is far from clear just what the electorate wants to do about these high costs. Lack of knowledge about the relation of variables other than cost to school size is a further limiting factor. It is also not clear that district size is the most important aspect of the situation; it could very well be attendance center size or educational program size. Finally, some schools will probably always be small no matter what the actions of the state government. The educational opportunities of children in these "forever small" schools cannot be ignored. (Author/IRT)