To determine the prevalence of the asymptomatic carriage of drug-resistant Streptococcus pneumoniae (DRSP) by children attending day-care centres in the Stockholm area, nasopharyngeal swabs were cultured from 1129 children and 308 day-care personnel in 36 day-care centres during a 3-week period, from March to April 1995. Approximately 36% of the children were asymptomatic carriers of S. pneumoniae sensitive to penicillin and other antibiotics. The highest prevalence of nasopharyngeal carriage was found in the 2-year-old group (50%), whereas among the 4-year-old children colonization was observed in 42%, and among the 7-year-old children 21% were asymptomatic carriers of penicillin-sensitive S. pneumoniae. In 2 day-care centres, 4 and 5 children, respectively, were found to have DRSP strains in the nasopharynx. The same serotype of DRSP strain was found among the children attending the same day-care centre. During the same period, none of the staff were found to harbour DRSP in the nasopharynx, but 3% were asymptomatic carriers of penicillin-sensitive S. pneumoniae. The patterns of nasopharyngeal colonization by Haemophilus influenzae, Moraxella catarrhalis and Group A streptococci were also studied in 635 children during the same period. 42% of the nasal cultures yielded Moraxella, 32% H. influenzae and 2% Streptococcus pyogenes.