Active prospective surveillance to identify the serotypes of Streptococcus pneumoniae responsible for invasive infections among children in Connecticut was conducted from July 1984 through March 1993. S. pneumoniae isolates (722) recovered from normally sterile sites (98% from blood, 7% from both blood and cerebrospinal fluid) were serotyped by the Quellung reaction; 70% of the isolates were from children < 2 years old. Serotype 14 caused 29% of the infections. Seven serotypes (4, 6B, 9V, 14, 18C, 19F, and 23F) were responsible for 84% of the invasive infections overall and for 86% of the invasive infections among children < 2 years old. Formulations of polysaccharide-protein conjugate pneumococcal vaccines designed to prevent invasive infections in children in the United States should include these commonly isolated serotypes.