BACKGROUND: Group B Streptococcus (GBS) is a leading cause of neonatal bacterial infections. Early-onset infections have decreased in recent years but, despite considerable efforts poured into prevention, cases continue to occur. OBJECTIVES: To analyze trends and identify determining factors for the persistence of the GBS infections. To evaluate the impact of antenatal screening and intrapartum chemoprophylaxis on the clinical presentation of the infection. METHODS: A prospective cohort, population-based study has been ongoing in Emilia-Romagna (Italy) since 2003. Invasive GBS infections, observed between 2003 and 2008 in infants aged < 7 days were analyzed. RESULTS: Among 214,120 live births, 61 early-infections were observed. Fourteen infants (23.0%) were born preterm. Among 47 infants who were delivered at term, 28 were born to mothers who had no risk factors and 7 were born to mothers who had none other than GBS colonization. Forty-one women at term had been screened prenatally; among them, only 10 were documented as GBS culture-positive.Disease severity was highest in infants at lower gestational ages, but most meningitis cases were observed in term infants born to mothers who were GBS culture-negative at screening.Nine newborns had culture-proven infection despite having received intrapartum antibiotics. They were born to mothers with > or =1 obstetrical risk factors and 5 mothers had been treated during labor with macrolides. CONCLUSION: Most infections presented in infants whose mothers had been screened as GBS culture-negative. Missed opportunities for prevention contributed more than prophylaxis failures to the early-onset disease burden.