Background: The concept of deterioration of nutrition status in inpatient children, also called nutrition deterioration (ND), has gained widespread importance and is described as significant weight loss during hospitalization periods. Our main purpose was to determine the incidence of ND in nonseriously ill children younger than age 5 years admitted as inpatients and explore the presence of associated factors. Materials and Methods : This descriptive study analyzed a prospective cohort of hospitalized children. We considered a significant weight loss of >2% or >0.25 SD in body mass index with respect to the admission weight. The correspondence between the 2 methods was assessed. The frequency of some clinical variables and its association with the outcome was explored. Results : The incidence of ND was 28.5% with at least 1 of the 2 methods of detection and was 20% with both definitions. The correlation between both methods was high (κ = 0.79). In children with ND, results were statistically significant in those with 5 or more stools a day, those hospitalized 5 or more days, and patients with lower respiratory tract disease. Conclusions : The incidence of ND was higher than that reported in the literature in children with nonserious disease. Diarrhea, lower respiratory tract disease, and hospital length of stay seem to be associated with these results. More studies are required to establish associations with those factors for an early detection of children at risk and for early interventions.