BACKGROUND The Acute Physiology and Chronic Health Evaluation (APACHE II) is the severity score and mortality estimation tool developed for use in ICU's around the world. It is calculated during the first twenty four hours of admission of the patient to an ICU. An integer score ranges from values 0 to 71. During the stay, the score is not recalculated; and by definition, is an admission score. METHODS This is a cross sectional study done on 100 patients admitted in the medical ICU in Sri Ramachandra Institute of Higher Education and Research. RESULTS Our results showed an APACHE II score of 16.5 which was significant and values equal to and higher than that correlated with the severity of illness. CONCLUSIONS APACHE II score is a good predictor of mortality in acute respiratory failure patients. It is a useful audit tool which helps the healthcare professionals to interpret and compare their care performance over time.