An epidemiological study of perinatal colonization of group B streptococcus (GBS) was carried out prospectively in a Chinese population. One hundred and sixty-eight mother-infant pairs were studied. Maternal carriage rate from multiple sites was 19.0%, with an overall neonatal colonization rate of 19.6%. There was no neonatal sepsis due to GBS. No obstetrical risk factors that could influence material carriage could be identified. Neonatal colonization was associated with premature/prolonged rupture of membranes, increased duration of rupture of membranes and intrapartum pyrexia. The number of infected sites and the density of organism in each site in the infant were directly proportional to the number and density of organisms in the culture-positive sites in the mother. Serotypical study showed a complete absence of type II GBS in this population. Since the vertical transmission rate was only 37.5%, it is postulated that nosocomial spread of the organism contributed significantly towards colonization of the infants in our study population.