Research into the effects of interventions on teacher efficacy is scarce. In this study, the long-term effects of an intensive data-based decision making intervention on teacher efficacy of mainly grade 4 teachers were investigated by means of a delayed treatment control group design (62 teachers). The findings showed significant strong intervention effects on teachers' efficacy for instructional strategies, and student engagement in both treatment groups. No significant effects were found for teacher efficacy regarding classroom management. Improved teacher efficacy in the first treatment group persisted throughout the second school year. Suggestions for future research are presented.