BACKGROUND: Group B Streptococcus (GBS) is known to be the leading cause of severe neonatal infections and also causes infections in pregnant women and adults with chronic underlying diseases. The frequency of GBS infections has increased recently. This study was aimed to determine the cultural results of beta-hemolytic streptococci and the clinical significance of the patients who had GBS infections during recent years. METHODS: This study has analyzed the isolation results and clinical importance of beta-hemolytic streptococci and GBS isolated from clinical specimens, except for stools, obtained from the patients in the Severance Hospital in Seoul from 1991 to 1999. RESULTS: Of 2,242 isolated beta-hemolytic streptococci, clinical records of 2,078 were available. GBS was found in 790 cases (38.0%). The isolation rate of group A Streptococcus was considerably high in pus and respiratory specimens while GBS was most commonly isolated from urogenital specimens. The isolation rate of GBS was much higher in females than males. GBS was the most common in patients under the age of one year or over age twenty. Especially, in newborn babies, GBS accounted for 72.7% of the total isolates. Of the 790 GBS isolates, 35.6% were considered to have definite infections. Of the 283 GBS infected patients, 56.2% had chronic diseases such as diabetes and malignancies. CONCLUSIONS: GBS is, two-fold, more commonly isolated than group A Streptococcus and the isolation rate is on the rise. GBS is more common in adult patients with chronic underlying diseases and remarkably high in newborn babies and the neonatal infections are severe. Therefore, microbiological diagnosis of GBS infections is necessary for proper treatment.