In a prospective study aimed at determining the etiology of community-acquired pneumonia in children nasopharyngeal cultures and paired serum samples were obtained from 336 consecutive children ages 6 weeks to 15 years with pneumonia, 167 hospitalized and 169 outpatients. Results regarding Haemophilus influenzae are reported here. Blood cultures obtained from 127 of the hospitalized patients did not yield growth of H. influenzae. H. influenzae was isolated from the nasopharynx of 88 children. Seventy-three strains were noncapsulated, 2 were type b, 2 were type f and 11 were not serotyped. Paired serum samples were available from 38 children with growth of noncapsulated H. influenzae in the nasopharynx as the only potential pathogen. Sixteen of them responded with significant increases in serum antibodies against outer membrane preparations prepared from their own nasopharyngeal isolates. Thirty-eight age- and sex-matched control children with pneumonia without growth of H. influenzae in the nasopharynx served as controls. Sera from each control patient were tested for antibodies against two strains of noncapsulated H. influenzae. Of those, 4 had significant increases in antibodies against one or both outer membrane preparations. The increases in serum antibodies against the outer membrane of noncapsulated strains of H. influenzae indicate that this organism might be a cause of pneumonia in some children.