The transition from preschool to elementary school requires high adaptive behaviours. Our study's aim is to analyze the reciprocal effects between self-regulation and behaviour problems during this transitional phase. Study results are based on longitudinal data collected at preschool and elementary school. Children's level of behaviour problems at school is predicted by preschool self-regulation skills and earlier behaviour problems. An optimal self-regulation is a substantial resource during the transitional phase and early interventions targeting at strengthening self-regulation skills are indicated.