Abstract. Objective: To validate two severity scoring systems, the Simplified Acute Physiology Score (SAPSII) and Acute Physiology and Chronic Health Evaluation (APACHEII), in a single-center ICU population. Design and setting: Prospective data collection in a two four-bed multidisciplinary ICUs of a teaching hospital. Patients and methods: Data were collected in ICU over 4years on 1721 consecutively admitted patients (aged 18years or older, no transferrals, ICU stay at least 24h) regarding SAPSII, APACHEII, predicted hospital mortality, and survival upon hospital discharge. Results: At the predicted risk of 0.5, sensitivity was 39.4% for SAPSII and 31.6% for APACHEII, specificity 95.6% and 97.2%, and correct classification rate 85.6% and 85.5%, respectively. The area under the ROC curve was higher than 0.8 for both models. The goodness-of-fit statistic showed no significant difference between observed and predicted hospital mortality (H=7.62 for SAPSII, H=3.87 for APACHEII; and C=9.32 and C=5.05, respectively). Observed hospital mortality of patients with risk of death higher than 60% was overpredicted by SAPSII and underpredicted by APACHEII. The observed hospital mortality was significantly higher than that predicted by the models in medical patients and in those admitted from the ward. Conclusions: This study validates both SAPSII and APACHEII scores in an ICU population comprised mainly of surgical patients. The type of ICU admission and the location in the hospital before ICU admission influence the predictive ability of the models.