Respiratory syncytial virus (RSV) is considered to be of a single serotype. Antigenic variants are detectable both by neutralization and monoclonal antibodies and have been divided into two broad categories, groups 1 and 2. Group 2 isolates have been considered to be uncommon. We used indirect immunofluorescence with strain-specific monoclonal antibodies to study RSV isolates from hospitalized infants in the greater Boston area. Of 223 RSV isolates recovered over a five-month period in 1983-1984, 125 (56%) were group 1, 92 (41%) were group 2, and 6 (3%) were of an intermediate character. Among 181 community-acquired RSV isolates, both temporal and geographic clustering was observed: group 1 isolates were common from January through March and predominated in central Boston; group 2 isolates were found principally in February and were acquired in outlying, particularly northern, areas. Strain-specific differences were not found with respect to sex, age, or clinical findings. An analysis of 82 RSV isolates from the 1981-1982 season showed 75 (91%) group 1 isolates and 7 (9%) group 2 isolates. We conclude that at least two antigenically distinct groups of RSV isolates may circulate concurrently in the community and that the prevalence of group 2 isolates appears greater than previously suspected.