Background: Existing diet quality indices often show theoretical and methodological limitations, especially with regard to validation. Objective: To develop a diet quality index based on the probability of adequate nutrient intake (PANDiet) and evaluate its validity using data from French and US populations. Material and Methods: The PANDiet is composed of adequacy probabilities for 24 nutrients grouped into two sub-scores. The relationship between the PANDiet score and energy intake were investigated. We evaluated the construct validity of the index by comparing scores for population sub-groups with 'a priori' differences in diet quality, according to smoking status, energy density, food intakes, plasma folate and carotenoid concentrations. French and US implementations of the PANDiet were developed and evaluated using national nutritional recommendations and dietary surveys. Results: The PANDiet was not correlated with energy for the French implementation (r = -0.02, P>0.05) and correlated at a low level for the US implementation (r = -0.11, P<0.0001). In both implementations, a higher PANDiet score (i.e. a better diet quality) was associated with not smoking, having a lower-energy-dense diet, consuming higher amounts of fruits, vegetables, fish, milk and other dairy products and lower amounts of cheese, pizza, eggs, meat and processed meat, and having higher plasma folate and carotenoid concentrations after controlling for appropriate factors (all P<0.05, carotenoid data for US not available). Conclusions: The PANDiet provides a single score that measures the adequacy of nutrient intake and reflects diet quality. This index is adaptable for use in different countries and relevant at the individual and population levels.