The purpose of this study was to assess the complex interaction of the heptavalent pneumococcal conjugate vaccine (PCV7) and individual classes of antimicrobial agents on the nasopharyngeal carriage of erythromycin-resistant pneumococci within the day-care center population. Between February 2005 and May 2007, nasopharyngeal cultures for Streptococcus pneumoniae were obtained from 1,829 day-care center attendees in Central Greece. Thirty-one percent of the pneumococci were erythromycin-resistant; 85.2% of these isolates were also penicillin-nonsusceptible. PCV7 immunization was associated with decreased carriage of erythromycin-resistant PCV7 serotypes but not with an overall decrease in erythromycin-resistant S. pneumoniae colonization. The largest decline in the carriage of erythromycin-resistant pneumococci, particularly of PCV7 serotypes, was observed among vaccinated attendees who had not been exposed to antimicrobials within the preceding 3 months. Exposure to macrolides, 90% clarithromycin, significantly correlated with erythromycin resistance (adjusted odds ratio [AOR] = 2.08, 95% confidence interval [CI] = 1.38-3.12). There was a trend toward an association between the use of oral cephalosporins, other than cefprozil and cefaclor, and colonization with erythromycin-resistant pneumococci (AOR = 1.91, 95% CI = 0.92-3.96). Penicillins had a nonsignificant correlation with the carriage of erythromycin-resistant pneumococci (AOR = 1.17, 95% CI = 0.80-1.71). Despite the widespread PCV7 immunization, the antibiotic pressure, particularly of macrolides, continues to cause dissemination of erythromycin-resistant, commonly multidrug-resistant, pneumococci within the day-care center population.