This study examines the effectiveness of a brief self-management intervention to support patients recently diagnosed with type-2 diabetes to achieve sustained improvements in their self-care behaviours. Based on proactive coping, the intervention emphasizes the crucial role of anticipation and planning in maintaining self-care behaviours. In a randomised controlled trial among recent screen-detected patients, participants who received the intervention were compared with usual-care controls, examining changes in proximal outcomes (intentions, self-efficacy and proactive coping), self-care behaviour (diet, physical activity and medication) and weight over time (0, 3 and 12 months). Subsequently, the contribution of proactive coping in predicting maintenance of behavioural change was analysed using stepwise hierarchical regression analyses, controlling for baseline self-care behaviour, patient characteristics, and intentions and self-efficacy as measured after the course. The intervention was effective in improving proximal outcomes and behaviour with regard to diet and physical activity, resulting in significant weight loss at 12 months. Furthermore, proactive coping was a better predictor of long-term self-management than either intentions or self-efficacy. Proactive coping thus offers new insights into behavioural maintenance theory and can be used to develop effective self-management interventions.