Community acquired bacterial pneumonia represents 8.7% of overall discharges of children under 15 years old in the Pediatric Hospital of the Centro Hospitalario Pereira Rossell; since 1997 treatment guidelines are being applied. Changes in etiology of this disease in children hospitalized in our institution from 1998 to 2004 were studied. Streptococcus pneumoniae was the most frequent agent in every year of the period, 91.4 +/- 2.3%; its susceptibility to penicillin has remained stable; serotypes 14, 1 and 5 were the most frequent isolated. In 2002 the first case of community acquired methicillin resistant S. aureus was identified; isolations of this microorganism were 5.8% in 2003 and 6.3% in 2004. Surveillance of etiology of community acquired pneumonia is mandatory in order to adjust guidelines to changes in agents and their antimicrobial susceptibility.