Streptococcus pneumoniae infection was indicated serologically in 84 (19%) of 449 children hospitalized with middle or lower respiratory tract infection. Pneumococcal antigen was detected in acute serum in 28 patients, but in acute urine in only 2. An antibody response to type-specific capsular polysaccharides of S. pneumoniae was indicated in 27 patients and to a protein antigen, pneumolysin, in 25 patients, but to C-polysaccharide in only 10 patients. The observations mentioned above suggest that each serological test for pneumococcal etiology is insensitive, and to get an optimal result, a large panel of pneumococcal antigen and antibody assays must be used. Pneumococcal infection could be indicated serologically although no focus of infection, such as pneumonia or acute otitis media, or no laboratory evidence of bacterial infection as elevated values of C-reactive protein concentration, erythrocyte sedimentation rate or white blood cell count was present. Particularly antibody responses to pneumococcal pneumolysin were present in children without pneumonia or acute otitis media. Our results point out that no nonspecific parameter can be used for the selection of patients with probable pneumococcal etiology among children with respiratory tract infection. Concomitant viral infection, in most cases RSV infection, was present in a third of the children with pneumococcal infection. It is concluded that pneumococcal etiology should be actively sought for also in patients with viral respiratory infection, especially in young children with RSV infection