Low-energy-density diets are often recommended for weight control. Such diets have a higher nutrient content than do high-energy-density diets. This study tested the hypothesis that energy-dense diets have a relatively low monetary cost, whereas less energy-dense diets are more expensive. In this cross-sectional study, dietary intakes of 1,474 French adults (672 men, 802 women), aged 15 to 92 years, were assessed using 7-day diet records. Dietary energy density (kcal/g) was calculated by dividing total dietary energy by the edible weight of foods and caloric beverages consumed. Diet cost ($/7 days or $/2,000 kcal) was estimated using mean national food prices for 895 foods. The relationship between dietary energy density and diet cost was examined in a linear regression model. Within each quintile of energy intakes, the more energy-dense diets were associated with lower diet quality and with lower diet costs (r²=0.38 to 0.44). In a regression model, the more energy-dense diets cost less, whereas low-energy-density diets cost substantially more, adjusting for energy intakes, sex, and age. The finding that energy-dense diets cost less per 2,000 kcal may help explain why the highest rates of obesity are observed among groups of limited economic means. The finding that low-energy-density diets are associated with higher diet costs suggests that lasting improvements in diet quality may require economic as well as behavioral interventions.