Purpose: To compare the predictive ability of the most used system scores Sequential Organ Failure Assessment (SOFA), Acute Physiology and Chronic Health Evaluation (APACHE) II and Simplified Acute Physiology Score (SAPS) II for mortality in mechanically ventilated (MV) patients in ICU (Intensive Care Unit). We conducted a single-center prospective observational study. It was realized in 8-bed ICU in a tertiary University Hospital in South Brazil during 12/04/17 - 05/04/18. In this period 85 patients were admitted in ICU and 60 patients were submitted to mechanical ventilation for 48 hours or more. Patients admitted to the ICU submitted to mechanical ventilation for 48 hours or more during this period were included. Patients not submitted to MV or with less than 48 hours were excluded. 60 patients were in MV for 48 hours or more, however 2 of them were transferred to another ICU and thus was not included in this study. Data were collected daily, by a single researcher, and in a longitudinal way to obtain variables of the medical, physiological and laboratory history. Database was divided in variables of patients' comorbidities: chronic obstructive pulmonary disease, chronic cardiac arrhythmia, ischemic heart disease, sleep apnea syndrome, asthma, neuromuscular and chest wall diseases, previous infections, neoplasia, sepsis, stroke, diabetes, other endocrine diseases; physiological variables: vital parameters (respiratory rate, pulse, mean artery pressure, axillary temperature), oxygenation parameters (PaO2/FIO2 ratio, PCO2, AaDO2, arterial pH, mechanical ventilation, Non-invasive ventilation) and acute physiological factors (use of vasopressor/inotropic, ARDS, dialysis, urine output, water balance, diet, Glasgow coma scale); laboratory variables: blood count, cultures, bilirubins, creatinine, urea, potassium, sodium, bicarbonate, albumin, glucose. It was also collected by the researcher daily the appearance of the following complications: postoperative infections in the ICU (pneumonia, abdominal and urinary sepsis and wound infections), hematological complications (thrombocytopenia <100,000) and renal complications (urine output <500ml or serum creatinine >170 μmol/l or dialysis for acute renal failure). Demographic data, diagnosis of patient admission, duration of ventilation mechanics, length of stay in the ICU and total hospital stay, survivors, non-survivors were obtained and classified within the study. The value of variables that are part...