OBJECTIVES: To assess the incidence of invasive pneumococcal disease (IPD) in all children younger than 5 years of age in the Northern California Kaiser Permanente (NCKP) health care system during a 5-year surveillance period (2000-2005) after the introduction in April 2000 of routine use of 7-valent pneumococcal conjugate vaccine (PCV7). METHODS: This was a laboratory-based surveillance study of all children younger than 5 years of age in the NCKP health care system from April 2000 to March 2005. The comparison group was all children younger than 5 years of age in the NCKP health care system from April 1996 to March 2000. Data obtained from clinical databases included microbiologic identification and susceptibility testing; serotyping of isolates; immunization records; and IPD diagnoses for inpatients and outpatients. IPD was defined as a positive culture of Streptococcus pneumoniae from a normally sterile body site. RESULTS: For all serotypes, the mean annual incidence of IPD during the postlicensure surveillance period was 15.3 cases/100,000 person-years (10(5) p-y) compared with 62.5 cases/10(5) p-y in the prelicensure years of 1996-2000. The average incidence of IPD caused by vaccine serotypes was reduced from 50.1 cases/10(5) p-y during the prelicensure years to 4.9 cases/10(5) p-y during the postlicensure period. The average incidences of IPD caused by cross-reactive and by nonvaccine serotypes were 5.8 and 5.3 cases/10(5) p-y, respectively, during the prelicensure years and 2.5 and 6.2 cases/10(5) p-y, respectively, during the postlicensure period. Of the 131 IPD cases observed during the postlicensure surveillance period, bacteremia (50.4%) and pneumonia (31.3%) were the most common diagnoses. During the 5-year postlicensure surveillance period, only 3 subjects who were identified to be fully vaccinated for age with PCV7 (3 doses by 7 months of age or 4 doses by 18 months of age) developed vaccine-serotype IPD. CONCLUSION: The incidence of IPD has significantly decreased in a large population of children after the introduction of PCV7. Vaccine-type IPD was rare in patients who received full 4-dose immunization with PCV7. There is no clear evidence of a significant increase in nonvaccine-serotype IPD. Introduction of a 4-dose infant schedule of PCV7 into this population has resulted in a marked and sustained reduction of IPD in children.