Streptococcus pneumoniae (pneumococcus) is a major cause of meningitis, pneumonia, and bacteremia, especially among young children and older adults. Before the 7-valent pneumococcal conjugate vaccine (PCV7) was introduced in the United States in 2000, the seven pneumococcal serotypes covered by the vaccine (4, 6B, 9V, 14, 18C, 19F, and 23F) caused 80% of invasive pneumococcal disease (IPD) cases among young children, and the incidence of IPD was relatively stable. In October 2000, the Advisory Committee on Immunization Practices recommended PCV7 for all children aged <2 years and for older children at increased risk for IPD. Introduction of PCV7 in the United States led to substantial reductions in the incidence of IPD among the target population of children aged <5 years. Use of the vaccine also reduced IPD among unvaccinated populations through reductions in nasopharyngeal colonization and transmission of vaccine-type pneumococci from vaccinated children (i.e., indirect, or herd, effects of PCV7). To evaluate the effect of continued PCV7 use on IPD incidence among children aged <5 years in the United States, CDC analyzed population- and laboratory-based surveillance data. Results of that analysis indicated that in 2005, overall IPD rates among children aged <5 years were 77% lower, and an estimated 13,000 fewer cases of IPD occurred, compared with the years preceding vaccine introduction (1998-1999). Although IPD caused by PCV7 serotypes declined through 2005, overall IPD rates leveled off beginning in 2002, primarily because of increases in the incidence of IPD caused by non-PCV7 serotype 19A. Given these trends, use of expanded-valency conjugate vaccines might further reduce IPD incidence. Continued surveillance is needed to guide development of future formulations of conjugate vaccines and to monitor the effects of continued vaccine use.