Undernutrition and its numerous clinical consequences increase morbidity and mortality. For these reasons, undernutrition screening and its treatment must be undertaken as soon as possible for every patient at risk. However in case of obesity, the criteria for the nutritional assessment are deficient because of the clinical and biological specificities associated with excessive body fat. The purpose of this review is to discuss the limits of the parameters actually used for nutritional evaluation in order to better assess its frequency and define the situations at risk of undernutrition in obese patients.