Community-acquired pneumonia (CAP) is common and potentially serious infection that affects children throughout the world. The objective of this study was to determine the etiology of CAP in children less than five years old. A prospective study was performed in Specialized Pediatric Hospital, Cairo University, from December 2006 to December 2007. The study included 428 children less than 5 years old with lower respiratory tract infections (LRTIs); visiting the outpatient clinic or recently admitted to the Children Hospitals. Following informed consent, the enrolled children provided NPA and blood specimen for culture and serology. NPA was used for detection of viral agents by DFA. Cases positive for influenza A virus were subsequently tested by PCR for detection of H5N1. The serum of the blood was used for detection of antibodies to atypical respiratory agents Chlamydophila pneumoniae and Mycoplasma pneumoniae by ELISA. The typical bacterial agents were detected by conventional bacteriological culture of NPA. Blood was processed for detection of invasive respiratory pathogen by blood cultures. The etiology of CAP was studied over one year. We detected by our methods; at least one respiratory pathogen in 264 (61.6%) out of 428 children with LRTIs. The detected respiratory pathogens were viral and bacterial agents (typical and atypical). The CAP caused by either single or multiple pathogens, as co infection is not uncommon. In the current study we detected 90(21%) viral respiratory pathogens, 96(22.4%) atypical respiratory pathogens and 156(36.4%) typical bacterial agents out of 428. We couldn’t detect any pathogen by our methods in 164 out of 428(38.3%). The most common respiratory pathogen detected was RSV in 71(16.6%) and S.pneumoniae was the most common typical bacterial agent in 99(23.1%).