The study sample consisted of all of the inpatient admissions and ambulatory visits (outpatient clinic and emergency department) from January 1, 1997, to December 31, 2004. An all-cause pneumonia–related hospitalization was defined as a pneumonia ICD-9 code listed as the primary (discharge) diagnosis (003.22, 011.6, 055.1, 073.0, 115.15, 115.95, and 480-487.0) or as a meningitis or septicemia ICD-9 code as the primary diagnosis and a pneumonia ICD-9 code as the secondary diagnosis (003.21, 013.0, 036.0, 036.1, 047, 049.0, 049.1, 053.0, 054.72, 072.1, 090.42, 091.81, 094.2, 098.82, 100.81, 112.83, 114.2, 115.01, 115.11, 115.91, 130.0, 320, 321, 322, 003.1, 020.2, 022.3, 031.2, 036.2, 038, 054.5,785.52, and 790.7). Some of these hospitalizations included a Current Procedural Terminology code indicating that radiography was performed. Pneumococcal pneumonia–related hospitalizations were identified by a specific pneumococcal pneumonia ICD-9 code or by unspecified pneumonia codes with other codes indicating pneumococcal disease. We excluded all of the pneumonia hospitalizations associated with birth (ie, the inpatient admission record included both pneumonia and delivery ICD-9 codes). If a patient with pneumonia had an ambulatory visit that later resulted in hospital admission, this was counted as 1 pneumonia hospitalization and 1 pneumonia ambulatory visit. To capture complete medical care cost information for ambulatory visits, we did not distinguish between first and follow-up visits in our analysis. We estimated the number of prevented pneumonia-related hospitalizations by the end of 2004 by multiplying the estimated rate differences by the specific 2004 population estimate from the census. Dehydration, a common cause of hospital admission in children,23 was used as a control condition to assess possible changes in admission practices.