Objective: To determine the carrier rate of group B beta-haemolytic streptococci (GBS) of pregnant women in Iceland and the colonisation of their newborns. Material and methods: A prospective study was conducted from October 1994 until October 1997, where culture specimens for GBS were taken from vagina and rectum of pregnant women attending the prenatal clinics at the Department of Obstetrics and Gynecology, Landspitali University Hospital and the Reykjavik Health Centre. The samples were taken at 23 and 36 weeks gestation and at delivery. Culture samples were also taken from axilla, umbilical area and pharynx of their newborn infants immediately after birth. Included in the study were pregnant women born on every fourth day of each month. Carrier state was not treated during pregnancy, but Penicillin G was given i.v. at delivery if the last culture before delivery was positive and gestational age was 12 hours before delivery or the mother had a fever >38 degrees C. Results: Cultures were taken from 280 women and their children. GBS carrier rate of pregnant women in Iceland was 24.3%. Twelve newborns had GBS positive cultures. No newborn had a confirmed septicemia. Cultures from 25% of newborns, who s mothers were still GBS carriers at birth, were positive for GBS. Positive predictive value of cultures taken at 23 weeks gestation was 64% and 78% at 36 weeks. Negative predictive value was 95% and 99% respectively. Conclusion: One out of every four pregnant women in Iceland is a GBS carrier. Twentyfive percent of newborns become colonised with GBS if the mother is a GBS carrier at delivery. When screening for GBS carrier state is done cultures from both vagina and rectum is more sensitive than cultures from vagina only. At least five percent of all newborns in Iceland are therefore expected to have positive skin cultures at birth. If the mother does not have positive GBS cultures during pregnancy, the likelihood that she will give birth to a GBS colonised child is almost none.