The serotypes of 474 clinically significant Streptococcus pneumoniae respiratory isolates collected during a national surveillance study in 1987-1988 were compared to the capsular types included in the 23-valent pneumococcal polysaccharide vaccine licensed for use in the United States. Overall, 355 isolates (74.9%) belonged to types included in the current vaccine, while another 65 (13.7%) were types serologically related to vaccine types and likely to be protective by virtue of cross-reactivity. Relatively few isolates (9.1%) belonged to nonvaccine serotypes, and only 2.3% were nontypable. The mucoid serotype 3 was most frequent (13.1% of total), followed by 19F (9.3%), 23F (7.4%), 6B and 14 (5.7% each), and 4 and 6A (5.5% each). The most frequent type not included in the vaccine was type 16 (2.1% of all isolates). Thus, nearly 89% of respiratory isolates included in this study were encompassed within the antigenic spectrum of the currently marketed pneumococcal vaccine.