This study was performed to investigate the viral etiological agents, age distribution and clinical manifestations of lower respiratory tract infection (LRTI) in hospitalized children. The viral etiology and clinical findings in 147 children (1 month to 5 years of age) hospitalized with acute LRTI were evaluated. Cell culture was used for isolation of influenza viruses and direct fluorescent antibody assay for parainfluenza viruses (PIVs), respiratory syncytial virus (RSV) and adenoviruses (ADVs). Reverse-transcriptase polymerase chain reaction was employed for human metapneumovirus (hMPV). One hundred and six of all patients (72.1%) were male, and 116 children (79.8%) were < or = 2 years. A viral etiology was detected in 54 patients (36.7%). RSV was the most frequently isolated (30 patients, 55.6%), and PIV (27.8%), hMPV (13%), influenza-A (9.3%), and ADV (5.6%) were also shown. Dual infection was detected in six patients. There were no statistically significant differences between the two groups (with isolated virus or no known viral etiology) with respect to symptoms, clinical findings, laboratory work-up, or radiological data. Length of hospital stay was also not different. Determination of the etiology of acute LRTI in children less than 5 years of age seems impossible without performing virological work-up, whether viral or nonviral in origin.