Purpose: To establish the association between caloric debt and loss of muscle mass measured by ultrasonography in adult patients hospitalized in the intensive care unit (ICU). Method: Observational prospective cohort study, 101 patients included, with a stay of not less than 7 days in the ICU. With a consecutive sample selection process. A total of 3 quadriceps thickness measurements were performed by ultrasonography on days 1, 3, 5 and 7. This muscle thickness was measured with a 10 MHz linear transducer by a trained physician. Two measurement sites were evaluated: at the midpoint or two-thirds of the length between the anterior superior iliac spine and the superior border of the patella. The variables evaluated were:Muscle thickness of the quadriceps, daily percentage of fulfillment of caloric and protein goals. Key findings: The thickness of the quadriceps femoris decreased by up to 20% during the first week after admission to the ICU. No correlation was found between muscle loss and caloric or protein debt during the ICU stay. Conclusions: The ultrasound evaluation of the thickness of the quadriceps can be considered as a tool to evaluate the loss of muscle mass in critically ill patients.