The assessment of severity is one of the most important issues in the management of the patient with community-acquired pneumonia. If forms the basis of decisions about hospitalization or admission to an intensive care unit. Age, comorbid illness and vital sign abnormalities have been shown to represent the principal criteria of pneumonia severity. Severe community-acquired pneumonia is characterized by one or more of the following criteria: acute respiratory failure, haemodynamic compromise, severe sepsis and septic shock, multilobar radiographic infiltrates, plus some additional laboratory parameters (blood urea nitrogen > 7 mM, lactate dehydrogenase > 260 U.L-1 and low serum albumin at admission). Several sets of corresponding simple clinical and laboratory criteria have consistently been shown to have considerable potential in predicting death caused by pneumonia. It was recently found that the tentative definition of severe community-acquired pneumonia provided by the American Thoracic Society guidelines is highly sensitive but poorly specific. An alternative rule, defining severe pneumonia as the presence of two of three minor criteria (systolic blood pressure < 90 mmHg, multilobar involvement and arterial oxygen tension/inspiratory oxygen fraction < 250) or one of two major criteria (mechanical ventilation and septic shock), had a sensitivity of 78%, a specificity of 94%, a positive predictive value of 75% and a negative predictive value of 95%. When validated in an independent patient population, this rule may contribute to a more uniform definition of severe community-acquired pneumonia.