The diagnosis of a pneumonia which occurs in critically ill patients undergoing positive pressure mechanical ventilation (ventilator-associated pneumonia, VAP) is often a problem. This is mainly due to the lack of sensitivity and specificity of clinical and radiographic signs of pneumonia in this patient population. Many studies investigated some clinical variables (fever, tracheal aspirates, blood leukocytosis, radiographic criteria): none of these, individually considered, resulted predictive enough to be useful for the bedside diagnosis of VAP. The Clinical Pulmonary Infection Score (CPIS) developed in 1991, based on 6 variables (fever, leukocytosis, tracheal aspirates, oxygenation, radiographic infiltrates, and semi-quantitative cultures of tracheal aspirates with Gram stain) is more sensitive to diagnose VAP. Compared with other associations of clinical variables this one is more flexible and it allows for the signs not to be all present at the same time.