OBJECTIVE The aim of this study was to assess the relationships of quantitative salivary levels of Streptococcus mutans and S. sobrinus in mothers with the colonization of mutans streptococci (MS) in plaque and caries status in their 2.5-year-old children. Furthermore, the dynamics of caries status in the children was evaluated in a 2-year follow-up survey. METHODS After oral examination of 54 mother-and-child pairs, the saliva samples from the mothers and the plaque samples from the children were collected. The levels (log DNA copies/ml saliva) of S. mutans and S. sobrinus were quantified using real-time polymerase chain reaction (PCR) assays, while MS in the plaque samples were detected using a cultivation method. In addition, 50 of the 54 children participated in a 2-year follow-up survey of caries prevalence. RESULTS In the 2.5-year-old children, the percentage of dft-positive subjects and mean number of dft were significantly higher in the MS(+) group when compared with the MS(-) group. Findings from the 2-year follow-up survey indicated that MS(+) subjects had a persistently higher mean number of dft at 4.5 years. The 2.5-year-old children were divided into three groups based on the quantitative levels of salivary S. mutans and S. sobrinus in their mothers: those whose mothers had low levels of S. mutans (<4 log DNA copies/ml) and S. sobrinus (<2) (group 1); those whose mothers had a high level of S. mutans (> or = 4) and low level of S. sobrinus (<2) (group 2); and those whose mothers had high levels of both (> or = 4 and > or = 2, respectively) (group 3). Among the three groups, the percentages of MS(+) and dft-positive children were highest in group 3 and lowest in group 1. Furthermore, multiple logistic regression analyses revealed that grouping the mothers based on salivary level of S. mutans and S. sobrinus was an efficient means to predict both MS colonization (OR = 2.96) and prevalence of dental caries (OR = 9.39) in children at 2.5 years of age. CONCLUSIONS In the 54 mother-and-child pairs tested, the maternal salivary levels of S. mutans and S. sobrinus determined by real-time PCR were significantly related to MS colonization in plaque as well as dental caries in their children at 2.5 years of age. Thus, determination of maternal levels of both organisms using the present cut-off values is proposed as an efficient method to indicate the risks of maternal transmission of MS and childhood dental caries.