BACKGROUND: The purpose of this study was to determine the incidence and seasonal distribution of viral etiological agents and to compare their clinical manifestations and disease severity, including single and co infections. METHODS: Multiplex reverse-transcription PCR was performed for the detection of viruses in nasopharyngeal aspirat. Disease severity was grouped using a categorization index as very mild/mild, and moderate/severe. Clinical and laboratory characteristics of hospitalized children with viral respiratory tract infection were analyzed. RESULTS: Viral pathogens were detected in 103/155 (66.5%) of patients. In order of frequency, identified pathogens were respiratory syncytial virus (32.0%), adenovirus (26.2%), parainfluenza viruses type 1-4 (19.4%), rhinovirus (18.4%), influenza A and B (12.6%), human metapneumovirus (12.6%), coronavirus (2.9%), and bocavirus (0.9%). Coinfections were present in 21 samples. Most of the children had very mild (38.8%) and mild disease (37.9%). Severity of illness was not worse with coinfections. The most common discharge diagnoses were "URTI" with or without LRTI/asthma (n=58). Most viruses exhibited strong seasonal patterns. Leukocytosis (22.2%) and neutrophilia (36.6%) were most commonly detected in patients with adenovirus and rhinovirus (p<0.05). Monocytosis was the most remarkable finding in the patients (n=48, 53.3%), especially in patients with adenovirus (p<0.05). CONCLUSIONS: RSV and RhV were associated with higher severity of illness in hospitalized children. RSV found to account for half of LRTI hospitalizations. In AdV and FluA and B infections, fever lasted longer than in other viruses. Coinfections were detected in 21 of the patients. The presence of coinfections was not associated with increased disease severity.