Our previous study reported that the fidelity of mutans streptococci (MS) transmission from mother to infant was gender- and race-specific within a Birmingham, Ala., population. We hypothesized that fidelity might be a function of postnatal nurturing practices. The purpose of the present study was to investigate those factors that could be correlated with MS transmission among Chinese children whose nurturing histories were known. Forty-eight families with 2- to 3-year-old children were selected from two kindergartens of Beijing, China. A questionnaire concerning the childhood nursing practice was obtained from the parents. Dental caries status was examined, and bacterial samples were collected for all participants twice at 6-month intervals. An average of six isolates of MS was picked at random from selective medium from each individual at each visit. Chromosomal DNA fingerprints were performed for all MS isolates to determine the genomic similarity within each family and among individuals. The results showed that 66% of the children at 2-3 years of age harbored MS, and 46% were caries-active. Of those children, 70% were breast-fed. Among the children whose chromosomal DNA fingerprint genotypes of MS matched that of their mothers, 88% were breast-fed compared with only 12% who were not breast-fed (p = 0.03). We also found that children who were breast-fed for more than 9 months were likely to harbor strains of MS common to their mothers (p = 0.04) and experience more dental caries (dmft = 4.4) at 3 years of age compared with children who were breast-fed less than 9 months (dmft = 1.4, p = 0.04). The data suggest that breast-feeding, especially prolonged breast-feeding, may correlate with the fidelity of transmission and that prolonged breast-feeding may contribute to a higher caries rate.