This article contributes to research concerning the determinants of student mobility between public and private schools. The authors analyze a unique set of data collected as part of a new evaluation of Milwaukee's citywide voucher program. The authors find several important patterns. Students who switch from the private to the public sector were performing lower than their peers on standardized tests in the prior year. African Americans were disproportionately more likely to leave the private sector, as were students in schools serving proportionally more voucher students. The authors argue that although these results indicate that a large voucher program may provide an educational home for some students, it may not provide a long-term solution to those who are among the most disadvantaged. (Contains 16 notes, 8 tables, and 3 figures.)