The aim of this study was to assess the effects of implementation intentions on leisure-time physical activity, taking into account the stability of intention. At baseline (T(0)), 349 participants completed a psychosocial questionnaire and were randomly assigned to implementation intention or control condition. Three months after baseline assessment (T(1)), participants in the experimental group were asked to plan where, when, and how they would exercise. Leisure-time physical activity was assessed 3 mo. later (i.e., at 6-mo. follow-up; T(2)). The intervention had no significant effect on physical activity at 6-mo. follow-up. However, a significant interaction of group and intention stability was observed, with the effect of the intervention on behaviour statistically significant only among those with unstable intention. Intention stability thus moderated the effect of the intervention, i.e., the intervention was more successful among individuals who needed support to change (unstable intenders).1061147159