This paper examines the evaluation index APACHE II in the Intensive Care Unit.The work is divided into two parts.The first part describes the theory of the various prognostic systems, and briefly presents prognostic systems based on disease severity and outcome (such as Acute Physiology and Chronic Health Evaluation, the Simplified Acute Physiology Score and the Mortality Probability Model), organ dysfunction assessment systems (such as Multiple Organ Dysfunction Score, the Sequential Organ Failure Assessment and Logistic Organ Dysfunction System), and severity rating systems based on the nursing workload (such as the Therapeutic Intervention Scoring System, the Nine Equivalents of nursing Manpower use Score, and the Nursing Activities Score), and also the neurological Glasgow Coma scale.There follows a comprehensive presentation of the Acute Physiology and Chronic Health Evaluation (APACHE II) score.In the second part of the work we performed a retrospective analysis of record data from 56 patients in the ICU of the G. Papanicolaou General Hospital of Thessaloniki, in order to investigate the possible correlation between the APACHE-II score, the outcome of patients in the ICU, and the duration of hospitalization in the ICU.The results show that the index APACHE II is a significant predictor of the outcome and the best cut-off value for predicting the outcome is 23.5.On the other hand, the APACHE-II score does not appear to interpret significantly the length of stay in the ICU.