Purpose: The aim of the present study was to undertake a systematic review to investigate whether mutans streptococci levels are a strong risk indicator/factor for ECC, based on the current quality of the literature regarding the relationship between early childhood caries (ECC) and these microorganisms. Materials and Methods: The authors searched PubMed, Scopus and Cochrane Library databases for papers from 1951 to 2007. The minimal inclusion requirements were assessment of preschool children reporting mutans streptococci counts, mainly in saliva and biofilm samples, and caries assessment. Since the heterogeneity of the studies did not allow a meta-analysis (X2 test), a qualitative analysis was conducted. Results: The electronic search yielded 120 articles, but only 16 scientific papers were critically appraised. Of the 16 scientific papers included in the review, only one cross-sectional study achieved a high value of evidence. Conclusions: It was concluded that mutans streptococci levels are a strong risk indicator for ECC. However, further welldesigned longitudinal studies with high evidence values are required to confirm mutans streptococci levels as a significant ECC risk factor. Keywords: dental caries, primary dentition, preschool child, review, Streptococcus mutans