Background and Objectives: Mechanically ventilated patients often face progressive and rapid losses of body mass and muscle because of hypermetabolism and increased protein catabolism. To investigate the impact of adequate nutritional provision during the early phase of intensive care unit (ICU) admission on the clinical outcomes in patients with medical illnesses receiving mechanical ventilation support. Methods and Study Design: Two hundred and eleven mechanically ventilated patients admitted to a 30-bed medical ICU were included. Three groups, based on nutrition intake, were examined: adequate protein intake (aPI), n=34; insufficient protein intake/ adequate energy intake (iPI/aEI), n=25; insufficient protein and energy intake (iPI/iEI), n=152. Results: Patients' mean age was 65±14 years; body mass index, 22±4; Acute Physiology and Chronic Health Evaluation II score, 24±7. The aPI group had significantly lower rates of in-ICU (14.7%) and in-hospital (23.5%) mortality than patients with insufficient protein intake: in-ICU mortality, iPI/aEI, 36%; iPI/iEI, 44.1% (p=0.006); in-hospital mortality, iPI/aEI, 56.0%; iPI/iEI, 52.0% (p=0.008). In the multivariate analysis, the hazard ratios (95% confidence intervals) for 60-day survival were 2.59 (1.02-6.59; p=0.046) and 2.88 (1.33-6.26; p=0.008) for the iPI/aEI and iPI/iEI groups, respectively. Conclusions: Despite possible selection bias owing to the retrospective nature of the study, achievement of ＞90% of target protein intake was associated with improved ICU outcomes in mechanically ventilated critically ill patients, based on real-world clinical circumstances.