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Respiratory syneytial virus (RSV) has two major antigenic groups, A and B. There is disagreement as to whether or not there is a difference in the clinical severity of disease caused by the two RSV groups. This 3-year prospective study of infants and children with RSV-positive bronchiolitis examines the relative virulence of RSV Groups A and B and assesses the role that breast-feeding may have in modifying the clinical severity of infection. Clinical severity was graded I (ventilated, severe), II (oxygen therapy, moderate), III (no ventilation or oxygen, mild). RSV serogrouping was performed. After exclusion of 60 subjects with known predisposing factors for severity, 444 infants and children were studied (Group A, 337; Group B, 107). The difference in proportion of subjects with severity grade I and II with Group A compared with Group B infection was 6% (95% confidence interval, 4.7 to 16.7). For infants 0 to 6 months of age this difference was 5.6% (95% CI, 7.3 to 18.4). Nineteen cases were nosocomially transmitted. There was a predominance of Group A RSV infection but no difference in severity between Group A and Group B infection.