Viruses are the most common cause of lower respiratory tract infections (LRTIs) in infants and young children and are a major public health problem in this age group. Viruses were identified in 54.9-70.4% of hospitalized infants and children with LRTIs in Korea. The viral pathogens identified included respiratory syncytial virus (RSV) A and RSV B, influenza (Inf) A, Inf B, parainfluenza (PIV)1, PIV2, human bocavirus (hBoV), human rhinovirus (hRV), adenovirus (ADV), human metapneumovirus (hMPV), human coronavirus (hCoV)-OC 43, hCoV-229E, hCoV-NL63, hCoV-HKU1, and human enterovirus (hEV). Coinfections with > or =2 viruses were observed in 11.5-22.8% of children. The occurrence of LRTIs was the highest in the first year of life. The specific viruses are frequently associated with specific clinical syndromes of LRTIs. LRTIs caused by RSV were predominant among younger infants. hRV accounted for a larger proportion of LRTIs in young infants than ADV and hBoV. hMPV was frequently detected in children >24 months old. The number of hMPV infections peaked between February and May, whereas hRV was detected throughout the year. Thus far, hCoV is a less common respiratory pathogen in cases of ALRI and URI in Korean children.