Workplace violence is a major occupational hazard for healthcare workers, generating a need for effective intervention programs. The purpose of this study was to evaluate the effectiveness of an aggression management training program. The evaluation design was based on the internal referencing strategy, an unobtrusive and applicable evaluation method that rules out some major threats to internal validity without the need for a control group. On three occasions, training participants completed a questionnaire containing experimental and control variables. As hypothesized, there was a significant improvement in the experimental variables that was larger than the non-significant change in the control variable. We conclude that aggression management training may be an effective instrument in the fight against workplace violence.