The aim of this study was to assess the relationship between salivary mutans streptococcus (SMS) levels and the prevalence and incidence of dental caries in 148 children (mean baseline age 3.8 years) of low socioeconomic status. Caries data (dmfs) and semiquantitative SMS counts were obtained at baseline and annually for 2 years. The children were classified during each of the 3 years as low (0 colony-forming units; CFU), moderate (1-50 CFU), or high (> 50 CFU) caries risk based on total SMS counts. The results indicated that the prevalence of dental caries increased with SMS levels at baseline and generally in both assessment years. At baseline the mean dmfs of the children classified as low, moderate, or high caries risk was 0.15, 1.44, and 3.36, respectively, while the mean dmfs of the same children at year 2 was 1.18, 3.10, and 7.87, respectively. The mean dmfs of the high-caries-risk group increased by 79% between baseline and year 1 and by 30% between year 1 and year 2. At year 2, 50% of the children in the low- and 47% in the moderate-caries-risk groups were caries free as compared with 11% in the high-caries-risk group. Thirty-three percent of the children examined remained in the same caries risk category from baseline to year 2; the incidence (delta dmfs) of dental caries in those children designated as low, moderate, and high caries risk during this period was 1.44, 3.36, and 10.07, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)