Laboratory findings such as white blood cell (WBC) count, C-reactive protein (CRP) concentration and erythrocyte sedimentation rate (ESR) level in patients with bronchiolitis, bronchopneumonia and lobar pneumonia caused by respiratory syncytial virus (RSV) were studied. The diagnosis of having RSV infection of the lower respiratory tract was made on the presence of RSV antigen in nasopharyngeal specimens by means of enzyme immunoassay, on chest X-ray appearances and clinical manifestations. The WBC counts in the lobar pneumonia cases (n = 25, 12,288 +/- 6296/mm3) were significantly greater than those for the bronchiolitis (n = 52, 9562 +/- 2720/mm3) and bronchopneumonia (n = 43, 8369 +/- 3714/mm3) cases. The concentrations of CRP in lobar pneumonia cases (n = 25, 6.5 +/- 7.3 mg/dL) were significantly greater than those in the bronchiolitis (n = 52, 1.9 +/- 2.0 mg/dL) and bronchopneumonia (n = 43, 2.1 +/- 2.4 mg/dL) cases. The ESR levels in the lobar pneumonia cases (n = 24, 43.8 +/- 29. mm/h) were also significantly higher than those in the bronchiolitis (n = 34, 20.1 +/- 12.3 mm/h) and bronchopneumonia (n = 40, 24.7 +/- 15.9 mm/h) cases. There were no significant differences in the WBC counts, the CRP concentrations and ESR levels between the bronchiolitis and bronchopneumonia cases. These results suggest that the RSV lobar pneumonia cases are coinfected with some bacterial organisms more heavily than in the RSV bronchiolitis and bronchopneumonia cases.