This study evaluates the impact of the privatization of education services in the Philadelphia School District, using an interrupted time series design. The sample observes 88 middle-grades schools, beginning with the 1996-97 school year, and finds that, by 2006, four years postintervention, the achievement growth rate of schools run by educational management organizations (EMOs) had fallen significantly below the growth rate of district-run schools in both reading and mathematics, whereas, in 2002, immediately prior to privatization, those schools later run by EMOs had achieved growth rates significantly higher than the rest of the district in reading and statistically equivalent in mathematics. (Contains 2 tables and 6 figures.)