The effect of student mobility on achievement on the High School Proficiency Test (HSPT), a state-mandated graduation test in New Jersey, was studied. Students in a small urban district in metropolitan New York City frequently transfer into and out of the district or between schools within the district. The research was conducted in an urban high school with a total enrollment of 1,628. The sample consisted of 315 tenth graders who had taken the HSPT in April of 1986. Seven hypotheses were tested with respect to mobility, school, and home factors that may affect student achievement and influence student performance on the HSPT. Student transcripts provided some of the study data. The results indicate that: (1) mobility has little effect on achievement when other school and home factors are held constant; and (2) a large set of complex social conditions affects student performance including language proficiency, behavior, and attendance. (TJH)