Cardiovascular diseases represent a significant cause of morbidity and mortality worldwide, and an unhealthy lifestyle notoriously accounts for a large percentage of their risk. Identifying resources to stimulate lifestyle changes is an essential goal of primary and secondary cardiovascular prevention. Self-efficacy beliefs are among the major psychological factors proven to impact health status and lifestyle. This study aimed to confirm the role of self-efficacy beliefs by investigating their associations over 5 years of adherence to healthy lifestyles in terms of diet and physical activity in a sample of 275 newly diagnosed patients with acute coronary syndrome. Longitudinal profiles of lifestyles and self-efficacy beliefs in their improvement were identified through latent class growth analysis. Correlations were then performed to explore the associations between lifestyles and self-efficacy trajectories. Results showed a positive association between virtuous lifestyle profiles and high self-efficacy in implementing behavioural change. Finally, two logistic regressions were performed to test the hypothesis that a high self-efficacy profile would predict better lifestyles 5 years after the coronary event. This hypothesis was confirmed for diet. Overall, current findings confirm the importance of implementing repeated psychological interventions that promote patients' efficacy beliefs in self-regulating their behaviour changes over time.