We retrospectively assessed the medical records of pregnant women who delivered at Asahikawa Kosei Hospital during a period of 3 years between January 2009 and December 2011 and their neonates. Our prophylactic measures against group B Streptococcus (GBS) infection are based on the Japanese guidelines. More specifically, we performed screening by examining bacterial cultures of vaginal–perianal swabs from pregnant women between gestational weeks 33 and 37. Then, sulbactam/ampicillin (SBT/ABPC) was given at a dose of 1.5 g through a drip intravenous infusion at delivery if pregnant women were screened positive for GBS. For neonates born to GBS carrier women, bacterial cultures of pharyngeal swabs, vernix caseosa, stool, and gastric juice were performed at birth. There were 2,399 deliveries and 2,499 births at our hospital. In 169 of the deliveries (175 of the births), GBS was isolated from specimens obtained from gestational weeks 33–37. According to delivery mode, there were 42 cases of cesarean section (45 births) and 127 cases of vaginal delivery (130 births). The GBS-positive neonates accounted for 4.1 % of all deliveries in pregnant women who tested positive for GBS at gestational weeks 33–37. In neonates born by vaginal delivery, the GBS-positive rate was 5.5 %. Of the 2,499 neonates born at our hospital during a period of 3 years, early-onset GBS infection occurred in 1 neonate. The incidence of early-onset GBS infection was 0.40 per 1,000 live births. From 1997 to 2001 (routine GBS screening of mothers was not performed), there were 2,097 deliveries and 2,166 births. Early-onset GBS infection occurred in 1 neonate during this period; thus, the incidence of early-onset GBS infection was 0.46 per 1,000 live births. There were no significant differences in the two periods. The present prophylactic measures such as screening of maternal GBS carriers and intrapartum antibiotic administration are inadequate to decrease the occurrence of early-onset GBS infection.