In recent decades, magnet schools have become a way for districts to provide school choice. Magnet schools are one of the many options provided to parents so that they can select a school to meet their children's educational needs and interests. After a short gap in federal grants for magnet school implementation, Congress established the current Magnet School Assistance Program (MSAP) in 1984. This study examined 21 MSAP-supported elementary schools from around the nation to see how their student body composition and academic achievement changed over time. The group of schools contained 17 that converted to become what might be called "traditional" magnet schools and another 4 that converted to become "destination" magnet schools. This study relied on administrative data collected directly from the districts. This study found that schools were more diverse after conversion but the evidence that this was related to conversion was limited (only significant for the concentration of minority students in traditional magnet schools). Achievement was higher after conversion in traditional magnet schools. The comparative interrupted times series (CITS) analysis suggests that this rise in achievement may have been due to other factors such as the changing student population rather than the magnet program or new peers as this analysis did not find evidence that magnet conversion itself was associated with a change in neighborhood student achievement gains. However, there was a significant amount of variation across individual district results which suggests that converting magnet schools or their contexts may be so different that an average look across them (even separately by traditional versus destination) may be less meaningful, or at least that it would be important to study a larger number of these schools in the future. Tables and figures are appended. [For the full study, "What Happens When Schools Become Magnet Schools? A Longitudinal Study of Diversity and Achievement," see ED556800.]