Community acquired pneumonia (CAP) is an acute respiratory infection that affects pulmonary parenchyma, and is caused by community acquired microorganisms. In Chile, pneumonia represents the main cause of death due to infectious diseases and is the third specific cause of mortality in adults. In 1999, an experts committee in representation of "Sociedad Chilena de Enfermedades Respiratorias", presented the first National Guidelines for the Treatment of Adult Community Acquired Pneumonia, mainly based in foreign experience and documents, and adapted it to our National Health System Organization. During the last decade, impressive epidemiological and technological changes have occurred, making the update of guidelines for treatment of NAC by several international scientific societies, necessary. These changes include: new respiratory pathogens that are being identified in CAP and affect adult patients (Mycoplasma pneumoniae, Chlamydia pneumoniae, Legionella pneumophila); the increasing senescent adult population that carries multiple co-morbidities; the emergence of antimicrobial resistance among respiratory pathogens associated to massive antibiotic prescription; the development by the pharmaceutical industry of new drugs that are effective for pneumonia treatment (macrolides, ketolides and respiratory fluorquinolones); and the development of new diagnostic techniques for detection of antigens, antibodies, and bacterial DNA by molecular biology, useful in respiratory infections. Based on these antecedents, an Advisory Committee of "Sociedad Chilena de Enfermedades Respiratorias" and "Sociedad Chilena de Infectología" has reviewed the national and international evidence about CAP management in adults in order to update clinical recommendations for our country