Background: Low-energy-density diets are associated with higher diet costs per 10 MJ. Are diets of higher nutritional quality also associated with higher costs per 10 MJ? Objective: The objective was to determine the relations between energy-adjusted diet costs, dietary energy density, and nutritional quality of diets. Design: This was a cross-sectional study of a nationally representative sample of 1332 French adults in the 1999 Enquête Individuelle et Nationale sur les Consommations Alimentaires (INCA) data set. Analyses were based on data from 7-d food records. The monetary cost of each diet was estimated by using mean retail prices for 620 foods. Nutritional quality was estimated by calculating the mean adequacy ratio (MAR), based on 23 nutrients. Energy density was based on solid foods only. Results: In a bivariate analysis, low-energy-density diets were of higher nutritional quality but also cost more. Participants in the highest tertile of MARs had the lowest dietary energy density and the highest diet costs, calculated both per day and per 10 MJ. In a multivariate model, dietary energy density was negatively linked to diet costs ({euro}/10 MJ), whereas MAR values were positively linked to diet costs after adjustment for age and energy intake. For a given energy intake and energy density, each 10% increase in MAR led to a 13% increase in estimated diet costs per 10 MJ. Conclusions: In this study of self-selected diets of French adults, lower energy density and higher nutritional quality were associated with higher energy-adjusted diet costs. Higher-quality diets cost more not only because they have a low energy density but also because they are nutrient rich