The evaluation determined that the program had some mixed positive effects that varied by gender, ethnicity/race, and grade, but the program had no overall effect on student bullying victimization. When the analysis was stratified by ethnicity/race, reports of relational and physical victimization decreased by 28 percent and 37 percent under different measurement standards among White students relative to those in comparison schools that did not use the program. No similar effect was found for students of other races/ethnicities. Neither were there differences by gender or by grade. Students in intervention schools were more likely to perceive other students as actively intervening in bullying incidents, and sixth-graders were more likely to feel sorry for bullying victims and want to help them. A major implication of these findings is that schools with an ethnically diverse student body should not stop the program, but rather encourage positive ethnic identity while advocating the development of tolerance and sensitivity to other ethnic groups. The evaluation involved a controlled trial with a cohort of 10 middle schools (grades six through eight) following a statewide mandate that required all middle schools to implement antibullying policies and measures. Seven schools chose to implement the Olweus Bullying Prevention Program, and the remainder of the schools pursued less formal activities. Measures of outcome for the schools that used the program and those that did not were bullying victimization experiences, perceptions of safety, and support at school. Key informant interviews were conducted by the primary investigator at each of the schools. 1 figure, 4 tables, and 38 references