This was to study the longitudinal assessment of caries activity of Streptococcus sobrinus (SS) positive children during their mixed dentition.The occurrence of mutans streptococci (MS) in plaque and saliva was determined in a representative sample of 55 children aged 8 to 12 years over a period of 4 years. A total of 708 bacterial strains was isolated which were identified as MS or SS. Caries activity (DeltaD(1-4)MFS) as well as plaque and gingival inflammation were recorded.During the period of observation 52 of the 55 children harboured MS; 12 of these children were SS positive. SS was not permanently detectable and 3 of the children were MS and SS negative. SS was not found without the presence of MS. Children that were infected with both SS and MS showed a slightly higher increase in caries compared with children that were infected exclusively by MS (DeltaD(1,2)MFS 6.2 vs. 3.0 and DeltaD(3,4)MFS 5.3 vs. 3.8) over the period of 4 years. An SS infection accelerated the increase of DeltaD(3,4)MFS significantly by a factor of 4 one year after its detection, whereas the DeltaD(1,2)MFS was 3 times as high during the period of infection.The findings suggest that an SS infection represents an important additional risk factor for dental caries due to its obvious aggravating of caries activity.