This random assignment impact study of Big Brothers Big Sisters School-Based Mentoring involved 1,139 9- to 16-year-old students in 10 cities nationwide. Youth were randomly assigned to either a treatment group (receiving mentoring) or a control group (receiving no mentoring) and were followed for 1.5 school years. At the end of the first school year, relative to the control group, mentored youth performed better academically, had more positive perceptions of their own academic abilities, and were more likely to report having a “special adult” in their lives. However, they did not show improvements in classroom effort, global self-worth, relationships with parents, teachers or peers, or rates of problem behavior. Academic improvements were also not sustained into the second school year.