This research examined whether naturally occurring self-fulfilling prophecies influenced adolescents' responsiveness to a substance use prevention program. The authors addressed this issue with a unique methodological approach that was designed to enhance the internal validity of research on naturally occurring selffulfilling prophecies by experimentally controlling for prediction without influence. Participantswere 321 families whowere assigned to an adolescent substance use prevention program that either did or did not systematically involve parents. Results showed that parents' perceptions about the value of involving parents in adolescent substance use prevention predicted adolescents' alcohol use more strongly among families assigned to the prevention program that systematically involved parents than to the one that did not. The theoretical and practical implications of these findings are discussed.