OBJECTIVES: The aim of this work was to evaluate whether the short-term stability (repeatability) of dietary patterns derived using a priori and a posterior approaches is affected by food items or food groups selected for the analysis. A priori methods seek to determine a person's adherence to a pre-specified (usually 'healthy') diet, whereas a posterior methods are used to record a person's (or more generally a population's) actual diet. DESIGN AND MAIN OUTCOME MEASURES: To test the a priori approach, the MedDietScore was selected as an index of adherence to the Mediterranean diet. A sample of 500 participants (aged 18-82 years, 38% male) completed a 76-item food frequency questionnaire (FFQ), while another 250 participants (aged 17-76 years, 34% male) completed a 36-item FFQ and the MedDietScore, twice within a 15-day interval. The MedDietScore was additionally indirectly calculated from participants' 76-item FFQ and 36-item FFQ answers. To test the a posterior approach, principal-components analysis (PCA) was used to derive dietary patterns. PCA was applied to data derived from the 76-item FFQ, using as independent variables first the 50 food items recorded on the FFQ and second 12 food groups (i.e., aggregated data) derived from the FFQ items. RESULTS: Both the indirectly calculated (i.e., through the two FFQs) and the directly recorded MedDietScore mean values were similar between the two administrations (p-values>0.05). In addition, PCA using the 50 food items revealed three major dietary patterns that were similar in both administrations, explaining 23% and 25% of the dietary intake variability, respectively, while PCA using 12 food groups also revealed three similar patterns between the two administrations (explaining 43% and 46% of the dietary intake variability, respectively). CONCLUSIONS: The results indicate the short-term stability of dietary patterns derived using either a priori or a posterior methods. For the a posterior approach the use of food groups, instead of food items, seems to explain more variation in dietary intake.