This study investigates the effects of a feedback intervention employing text messaging during 11 weeks on adolescents' behavior, self-efficacy and outcome expectations regarding fruit and vegetable intake. A pre- and post-survey was completed by 1488 adolescents school-wise randomly allocated to a control group and two experimental groups. Both experimental groups set weekly goals on fruit and vegetable intake, reported their consumption daily and subsequently received feedback on their performance via mobile text messaging (Short Message Service [SMS]). The second experimental group also received, in addition, a 45-min nutrition education session from a dietitian during school. The direct effects of the interventions were not significant. However, for adolescents participating in the SMS routines, there were significant effects of the level of engagement in the intervention, reflected in the number of sent text messages, on intervention outcomes. Participants sending more than half of the possible text messages significantly increased their fruit and vegetable intake. Participants sending between 10% and 50% of the possible text messages experienced a significant drop in self-efficacy and those sending less than 10% experienced a significant drop in outcome expectations. The findings suggest that participants' active engagement in an intervention is crucial to its success. Implications for health-promoting interventions are discussed.