Various studies of prevalence in our hospitals have detected large percentages of under-nourished patients in both medical and surgical wards. These groups of patients are not detected in clinical practice and are therefore untreated, leading to nutritional deterioration. The present paper studies the prevalence of the risk of malnutrition, using the Cardona risk assessment sheet as modified by Mager, in 134 patients between the 3rd and 5th day after admission to hospital. A chi-squared test was applied to allow comparison of quantitative and qualitative variables. We found 56.70% of patients risked under-nourishment (60% in medical wards and 50% in surgical wards). The variables which showed the best correlation with the risk of malnutrition were albumen (p < 0.01), weight loss (p < 0.001) and age (p < 0.005). The Hospital's Clinical Nutrition Committee must define the standard to be achieved regarding nutritional risk in order to ensure the quality offered to our patients in this regard.