The authors aim to describe student attendance-mobility within a large urban district in ways that are meaningful and useful to schools and the community. First, the prevalence of mobility and nonattendance in Grades 1-12 across all students and by gender, ethnicity, and socioeconomic subgroups is presented. Second, the impact on student mathematics achievement is examined. Results show that nonattendance-mobility negatively impact mathematics achievement as measured by the state's assessment, even after controlling for socioeconomic status and gender. Interestingly, there is not a differential impact across ethnicities. Black and White subgroups show similar patterns of achievement across attendance and mobility levels. Finally, the authors take a closer look at the 10 district high schools to determine where nonattendance-mobility is of particular concern. Implications for districts are discussed in terms of targeting the extent of the problem and where it is occurring, using that information to improve attendance and reduce mobility, and finally, instituting systematic approaches to deal with student movement in and out of schools. (Contains 3 notes, 3 tables and 7 figures.)