This report provides reliable evidence that mentoring programs can positively affect young people. The evidence is derived from research conducted at local affiliates of Big Brothers/Big Sisters of America (BB/BSA), the oldest, best-known, and arguably most sophisticated of the country's mentoring programs. Public/Private Ventures, Inc. conducted a comparative study of 959 10- to 16-year-olds who applied to 8 local BB/BSA programs in 1992 and 1993. Half were randomly assigned to a treatment group with BB/BSA matches made or attempted, and half were assigned to BB/BSA waiting lists. Comparison after 18 months found that BB/BSA participants were less likely to use drugs and alcohol; were less likely to hit someone; had improved attendance, school performance, and attitudes toward completing school work; and demonstrated improved peer and family relationships. These results are clear and encouraging evidence about the importance of caring relationships between young people and adults supported by mentoring programs. Appendix A (five tables) describes study methodology, and Appendix B provides six additional tables of study findings. (Contains 16 text tables and 69 references.) (SLD)