We conducted a long-term follow-up study between December 2005 and February 2007 on 4 immunocompetent infants, who repeatedly presented with respiratory symptoms, using PCR-based techniques targeting 14 viruses related to acute respiratory tract infection. Of 38 specimens, 30 were collected from symptomatic infants and 8 were collected when respiratory symptoms were absent. Overall, one or more respiratory viruses were detected in 94.7% (36/38) of the specimens. Of the 36 PCR-positive specimens, 77.8% (28/36) were positive for more than one virus. Most of these co-infections were double infections (55.6% or 20/36). Of note, co-infections with 4 and 3 viruses were observed in 3 (8.3% or 3/36) and 5 (13.9% or 5/36) specimens, respectively. Of the 8 specimens collected from the 4 infants when apparent respiratory symptoms were absent, 7 (87.5%) were positive for respiratory viruses. Respiratory viral co-infections were also frequent and found in 5 of the specimens (62.5%). However, apparent correlation between disease severity and co-infection was undetectable due to the limit of the number of cases studied. Taken together, this longitudinal study revealed that respiratory viral co-infections were not infrequent in infants aged 0-2 years, regardless of the presence of respiratory symptoms (62.5-77.8%).