In this retrospective epidemiologic study, we present pneumococcal carriage data from 109 Swedish day care centers over a period of 10 years. Aspects of season, age, personnel and group size were studied. We found a significant seasonal variation in pneumococcal carriage. Group size was a significant risk factor for pneumococcal carriage. Pneumococcal carriage was 4.5 % in personnel. (author abstract)