In 2000, a heptavalent pneumococcal conjugate vaccine (PCV7) was licensed and recommended for all children aged 2 to 23 months. This study compares the rates of invasive pneumococcal disease (IPD) in young infants before and after the addition of PCV7 to determine whether the new vaccine has impacted pneumococcal rates in both the target and nontarget groups. The study design used a prospective, population-based study of infants aged 0 to 90 days from eight U.S. states that maintain laboratory surveillance for invasive S pneumoniae infections. Rates of laboratory-confirmed IPD were compared before (July 1997 to June 2000) and after (July 2001 to June 2004) PCV7 introduction. A transition year between 2000 and 2001 was excluded from the analysis.