Muscular atrophy implies structural and functional alterations related to the muscular force production and movement. This condition has been reported as the main reason for generalized muscle weakness; it reflects the severity of the disease and can have a profound impact onshort- and long-term clinical outcomes. The purpose of this study was to determine whethermuscle atrophy ultrasound arameters predict early muscle weakness, morbidity, or 28-daymortality. This was a prospective, observational single center cohort study. Ultrasound was used to determine the cross-sectional area (CSA) and muscle thickness (MT) of the rectus femoris onthe first and third day of ICU stay. The primary outcome was the incidence of significant muscleatrophy (¿10%). Ultrasound measurements were made in 31 patients, 59.6% of which shower significant muscle atrophy. We did not identify any predictors of significant muscle wasting, however. The 28-day mortality rate was similar in both subgroups. Clinically significant muscle wasting was frequent in our cohort of patients. The results could be used to plan future studies on this topic.