Respiratory syncytial virus (RSV) is the single most important cause of lower respiratory tract infection during infancy and early childhood worldwide including Thailand. The magnitude as well as the intensity of both infection and host response to the RSV infection determine the severity of disease. To determine the presence of anti-RSV IgG in children of different age groups, 124 serum samples were randomly collected from healthy children aged 6 months to 5 years. All of them were assayed for anti-RSV IgG using a commercial ELISA kit. The mean prevalence rate was found to be 68.91%. The prevalence increased with age: from 6 to 11 months, 11.76%; from 12 to 17 months, 41.67%; from 18 to 23 months, 60.87%; from 24 to 29 months, 88.24%; from 30 to 35 months, 78.57%; from 36 to 41 months, 94.44% and from 42 to 60 months, 100%. The amount of anti-RSV IgG among the groups was significantly different (p = 0.006). No sexual preponderance was found. RSV infection commonly occurred in children aged 12 to 29 months. All children older than 5 years had experienced a RSV infection.