Purpose <p>The aim of this study was to investigate the effect of mental adjustment to cancer on anxiety and depression levels in patients newly diagnosed with lung cancer.</p> Design and methods <p>This descriptive and correlational study was conducted with 150 patients receiving treatment at a university hospital. Data were collected using the Mental Adjustment to Cancer Scale and the Hospital Anxiety and Depression Scale. Statistical analyses included correlation and multiple regression analyses.</p> Results <p>The findings demonstrated that mental adjustment styles significantly influenced psychological outcomes. The fighting spirit subscale was strongly associated with lower levels of anxiety and depression, whereas anxious waiting was identified as a significant risk factor associated with increased psychological distress.</p> Conclusions <p>Mental adjustment to cancer plays a critical role in shaping psychological well-being during the early phase following diagnosis. Early identification of maladaptive coping styles may support timely and individualized supportive care interventions aimed at reducing anxiety and depression in patients with lung cancer.</p> Practice implications <p>This study demonstrated that mental adjustment to cancer plays a significant role in shaping anxiety and depression levels in patients newly diagnosed with lung cancer. The findings underscore the importance of early psychosocial screening to identify patients at risk and to guide individualized supportive care interventions delivered by multidisciplinary oncology teams, including oncology nurses. Integrating structured psycho-oncological assessments into routine cancer care may enhance clinical decision-making and improve psychological well-being during the early diagnostic period.</p>