Objectives: The purpose of this study was to investigate whether the objective medical parameters related to congenital heart disease and patients’ ratings of cardiac disease severity were related to psychological well-being and illness behaviour during the pre-operative period. Methods: A total of 143 patients (63 male patients; 16–73 years old) with congenital heart disease evaluated the severity of their cardiac condition using a numerical rating scale ranging from 0, indicating the least severe condition, to 100, indicating the most severe condition. Psychological well-being was assessed using the Psychological General Well-Being Index (total score ≤60 indicating severe distress) and illness behaviour using the Illness Behavior Questionnaire. Results: Pre-operative psychological well-being was not related to the objective medical parameters reflecting cardiac disease severity. In contrast, total psychological well-being scores correlated significantly with patients’ subjective ratings of disease severity (p < 0.001). When compared with the reference values from the Italian population, the mean scores of the patients on psychological well-being were similar. As regards the Illness Behavior Questionnaire, the scores on denial were higher and those on hypochondria were lower compared with other hospitalised patients. Conclusions: This study shows that the perception of cardiac disease severity, and not the medical parameters in congenital heart disease, is related to the patients’ pre-operative psychological state. Thus, more importance needs to be given to assessing the patients’ pre-operative perception and psychological state independently of cardiac severity. Targeted interventions with regard to the cardiac condition are recommended.