BACKGROUND: Multidrug-resistant Streptococcus pneumoniae occasionally cause treatment failure in various pneumococcal infection in children. The pneumococcal strains carried at the nosopharynx are frequently found at the time of invasive infection in one individual. We investigated the prevalence of nasopharyngeal carriage of S. pneumoniae and antibiotic susceptibility patterns among children attending day care center and compared the rates of antibiotic resistance of those strains with strains recovered from clinical specimens of patients at a tertiary care hospital. METHODS: Two-hundred nine children from 3 centers distributed at Jong-Ro area of Seoul, Korea were enrolled from May to June, 1998. Swabs were obtained from posterior nasopharynx and cultured onto tryptic soy agar containing 5% sheep blood without and with 10 microgram/mL gentamicin for 24 hours. Clinical isolates included the non-invasive isolates from sputum, ear discharge and sinus aspirate and the invasive isolates from cerebrospinal fluid and blood. Antimicrobial susceptibilities were determined with E-test and agar dilution method. RESULTS: The overall prevalence of nasopharyngeal carriage was 32.5%(68/209). The children >4, 5< OR = years of age have the highest carriage rate(36.7%). But there were no significant differences of prevalence in gender and centers. The prevalences of decreased antibiotic susceptibilities in the nasopharyngeal isolates and clinical isolates were, respectively: penicillin(75% vs 86%, NS); cefotaxime(20% vs 48%, p<0.05); multiresistance(57% vs 86%, p<0.05). Younger the age and female in gender, higher the resistance rate to penicillin. CONCLUSIONS: There was no significant difference in antimicrobial resistance of pneumococci between healthy nosopharyngeal carriages and clinical isolates causing invasive or non-invasive infections. The results acquired from this surveillence would provide a useful guide in an appropriate choice of antimicrobial agents for management of pneumococcal diseases.