Background: Cardiovascular diseases (CVDs) are a major cause of worldwide morbidity and mortality. Nutrition plays an important role in the primary, secondary, and tertiary prevention of CVDs. The present longitudinal study used the Health Action Process Approach (HAPA) to investigate changes in dietary behavior in coronary patients (CPs) affected by acute coronary syndrome and hypertensive patients (HPs) affected by essential arterial hypertension. Methods: CPs (N=250) and HPs (N=246) completed a questionnaire during three measurement points (baseline, 6-month follow-up, and 12-month follow-up). Statistical analyses included a repeated measures ANOVA and a multi-sample structural equation model. Results: HPs showed no changes in dietary behavior, whereas CPs improved their nutrition at 6 months and then maintained the healthier diet. The multi-sample analysis indicated equivalence of the HAPA model for both patient populations. Conclusions: These findings provide further evidencefor the generalizability of the HAPA model, shedding light on dietary behavior among CVD patients and particularly on hypertensive patients which received little attention. Moreover, the equivalence of the model suggests that the process of change is almost identical for individuals who are at high risk for coronary event (i.e., HPs) and individuals who have already had the event (i.e., CPs).