BACKGROUND Group B streptococci (GBS) may cause life-threatening invasive infections in infants. The incidence of these infections has been increasing during the last decades. The aim of the study was to determine the epidemiology of neonatal GBS infections to be able to implement therapeutic and preventive measures more effectively. METHODS A retrospective case study was conducted in Iceland that included all neonates with positive GBS cultures from blood or cerebrospinal fluid during the period 1975 to 2006. Serotyping of all available GBS isolates was performed. RESULTS A total of 87 children with 89 infections were included in the study. In all, 53 infants had early-onset (EO) GBS infections (occurring <7 days after birth) and 34 had late-onset (LO) infections (occurring on days 7-90). EO infections increased during the first 3 quartiles of the study period but decreased during the last quartile. LO infections increased throughout the entire study period. GBS was cultured from cerebrospinal fluid in 21 patients; 9 with EO and 12 with LO infections. Premature infants comprised 15 with EO and 14 with LO infections. Eight children died of GBS infection, 7 with EO and 1 with LO infections; no correlation with serotypes was found. Serotype III was most common for both EO (34%) and LO infections (62%). CONCLUSION The number of GBS infections increased during the study period. The decrease in EO infections in recent years could be attributed to intrapartum antibiotic treatment. The increasing number of LO infections is a concern.