The incidences and case-fatality rates from pneumonia in young children are much higher in Papua New Guinea than in developed countries. The most significant etiologic pathogens are Streptococcus pneumoniae and Haemophilus influenzae. The purpose of this study was to determine whether young PNG children have any abnormality of immune function which could predispose them to pneumonia. Pneumococcal antibody concentrations in pneumonia patients were compared with those in age-matched healthy Papua New Guinean and expatriate controls living in the same area