To study the molecular epidemiology of respiratory syncytial virus (RSV) in a community, children with acute respiratory symptoms at a pediatric outpatient clinic in Niigata, Japan, were analyzed over three seasons from November 2001 to July 2004. Of 499 nasopharyngeal aspirate specimens, 185 (37.1%) were RSV positive, and only 8 (4.5%) of 177 patients were shown by the reverse transcription (RT)-PCR method to be reinfected. RSV infection occurred beginning in the early winter, and the rates declined in the spring. The predominant subgroup changed from A to B and returned to A over the three seasons. Phylogenetic analysis also revealed that multiple genotypes cocirculated each year, with genotype GA5 of subgroup A predominating in the 2001-2002 and the 2003-2004 seasons. A new genotype of subgroup B (named BA, according to the nomenclature for viruses) with a 60-nucleotide insertion in the second variable region of the attachment glycoportein protein was predominant as an emerging strain in the 2002-2003 season, but this was not associated with new epidemiological or clinical features, unlike the cases of disease caused by other genotypes in the other seasons. In conclusion, our molecular analysis of RSV confirms that multiple genotypes cocirculate each year and that the genotype predominating may shift with the season. Support for determination of the genotype by RT-PCR as an effective tool for characterization of RSV circulation patterns in the community is provided.