Indirect immunofluorescence with strain-specific monoclonal antibodies was used to determine the phenotype of respiratory syncytial virus (RSV) isolates obtained from infants hospitalized in greater Boston over six successive outbreaks from 1981 to 1987. Of 981 isolates, 591 (60%) were classified as subgroup A and 383 (39%) as subgroup B. The prevalence of subgroups varied both between and within yearly outbreaks. In 1983-84 and 1984-85, both subgroups circulated concurrently and in almost equal proportions; in 1981-82, 1982-83, and 1985-86 subgroup A was dominant, accounting for 93% of all RSV isolates; and in 1986-87 subgroup B accounted for 89% of all RSV isolates. In some outbreaks both geographic and temporal clustering of subgroups occurred. No major differences in age, gender, or frequency of nosocomially acquired RSV between infants infected with either subgroup were seen, either overall or between or within yearly outbreaks. An expanded panel of monoclonal antibodies revealed further heterogeneity among subgroup A isolates. Comparison of these results with similar studies from other geographic locations indicated that the pattern of RSV subgroup prevalence is a localized phenomenon.