Personality Traits and Illness Acceptance in Patients with Cancer and Cardiovascular Disease: A Cross-Sectional Study from Poland

Background: Illness acceptance is a key psychological factor shaping adjustment to chronic disease, and personality traits are among its established correlates. Whether the diagnostic context—oncological versus cardiovascular disease—moderates the relationship between personality and illness acceptance remains insufficiently studied. This study examined associations between the Big Five personality traits (NEO-FFI) and illness acceptance (AIS) in patients with cancer and cardiovascular disease, and tested whether diagnosis moderates these associations. Methods: A cross-sectional study was conducted between January 2024 and May 2026 at the Oncology Centre of the Lublin Region, University Hospital No. 4, Lublin, and University Hospital No. 1, Lublin. The sample comprised 631 patients: 316 with a cancer diagnosis and 315 with cardiovascular disease. Personality traits were measured with the NEO Five-Factor Inventory (NEO-FFI) and illness acceptance with the Acceptance of Illness Scale (AIS). An unadjusted moderated multiple regression was used, with personality traits as predictors, diagnosis (0 = cancer, 1 = cardiovascular disease) as the moderator, and illness acceptance as the outcome; age, sex, and illness duration were not included as covariates, and all continuous variables were standardized. Analyses were performed in R (version 4.2.2) via RStudio (version 2022.12.0), with α = 0.05. Results: Patients with cardiovascular disease scored higher on neuroticism and lower on openness, agreeableness, and conscientiousness than patients with cancer (all p < 0.05); groups did not differ in extraversion or overall illness acceptance. The unadjusted moderated regression model was significant, F (11, 619) = 9.88, p < 0.001, explaining 14.9% of the variance in illness acceptance (adjusted R2 = 0.134). The diagnosis coefficient was positive and statistically significant in the unadjusted model (β = 0.23, p = 0.002), while neuroticism was negatively associated with illness acceptance (β = −0.29, p < 0.001). A significant openness × diagnosis interaction was observed (β = −0.22, p = 0.008); the model-estimated conditional coefficients for openness were β = 0.09 in the cancer group and β = −0.13 in the cardiovascular disease group. Formal inferential tests of the within-group slopes were not performed, and the model was not adjusted for age, sex, or illness duration. Conclusions: In this cross-sectional study, neuroticism was negatively associated with illness acceptance in the unadjusted model, whereas the openness × diagnosis interaction was exploratory. The conditional coefficients pointed in opposite directions across diagnostic groups, but the within-group slopes were not formally tested and the model was not adjusted for age, sex, or illness duration. These findings are hypothesis-generating and require confirmation in adjusted and longitudinal studies before informing diagnosis-specific clinical support.