All serotypes of group B streptococci (GBS) may cause early-onset(EO) infection in neonates. Type la has predominated in blood isolates and type III in CSF isolates. Recently, type V has been isolated from neonates with EOGBS disease. The distribution of serotypes causing EOGBS infection varies with time and geographic location. We describe the distribution of serotypes of GBS from 67 infants with early-onset sepsis systematically collected in thirteen hospitals of six academic institutions across the U.S from July 15, 1995 to February 5, 1997. Our data supports the previous reports that type Ia and III are the most common serotypes and that type V (15%) has emerged as a significant type for early-onset systemic neonatal infection. However, the frequency of the type V strain varies from site to site and is significantly higher in New York (31%) and New Jersey (25%) than the southern states (Alabama 9%, Florida 0%, Texas 15%) and northern California (7%) where type in is more prevalent. While type la (40%) is the predominant serotype in blood isolates in our series, the proportion of type la among the 6 study sites varies from 15% in New York to 64% in Alabama. Similarly, while type V (31%) is the predominant serotype in New York, type HI predominates in northern California and Florida (47% and 43%, respectively). The geographical differences in the distribution of serotype V may be due to chance alone since the number of cases observed at each site is relatively small. Types Ia, III and V accounted for 82% of the strains. The proportion of types Ia, III and V together ranges from 62% to 100% among the study sites. Two strains were non-typable indicating that newer serotypes might be emerging. Our observations demonstrate that the data on serotype distribution based on a single geographic location, or based on a small number of cases may not always be applicable for other locations, that types Ia, III and V represent important serotypes, and that continual monitoring of the serotype distribution is necessary for the design of a GBS vaccine.