Facing substantial financial pressure, many districts close schools in order to preserve solvency and improve student outcomes. Using a new longitudinal data set on all non-Cook County Illinois schools, we examine the determinants of high school closure decisions from 1986 through 2006. Our dataset combines information from a wide variety of sources, including the Illinois State Board of Education, the Census Bureau, the Illinois Department of Revenue, and the Bureau of Labor Statistics. Prior studies in this area typically use cross-sectional data, short panels, or case-studies. Enrolments and fiscal resources are indeed the most important determinants of school closures, but not always in expected ways. We carefully investigate the causes of closures, incorporating numerous trends and providing a series of robustness tests. Per-pupil expenditures, student demographics, and test scores are not significant determinants of closures. The most important predictors appear to be enrolment, school-aged population, and the proportion of property tax revenues allocated to the district. (Contains 6 tables, 1 figure and 9 footnotes.)