The New York City Leadership Academy represents a unique experiment by a large urban school district to train and develop its own school leaders. Its 14-month Aspiring Principals Program (APP) selects and prepares aspiring principals to lead low-performing schools. This study provides the first systematic evaluation of achievement in APP-staffed schools after 3 or more years. We examine differences between APP principals and those advancing through other routes, the extent to which APP graduates serve and remain in schools, and their relative performance in mathematics and English language arts. On balance, we find that APP principals performed about as well as other new principals. If anything, they narrowed the gap with comparison schools in English language arts but lagged behind in mathematics. (Contains 2 figures, 9 tables, and 23 notes.)