The relationship between the presence of Streptococcus mutans and caries on approximal tooth surfaces was studied in a group of 28 children who were 13 years old at baseline. 700 surfaces in the pre-molar and molar region were studied. Sampling of S. mutans was performed with a toothpick method at the beginning of the study and after one and two years. From bite-wing radiographs, the approximal surfaces were recorded as caries-free, decayed, or restored. Progression of lesions during the two years was also studied. More new caries lesions and progression caries lesions were found on surfaces positive for S. mutans than on surfaces without, and the more S. mutans, the more lesions, 52% of the surfaces with positive S. mutans findings at all three sampling occasions became carious or had caries in a progressive form. Among the surfaces with no detectable S. mutans, the corresponding figure was 8%. 80 out of the 530 surfaces diagnosed as intact at the start of the study became carious over the two-year period. 69 of these were S. mutans-positive at the first and/or second sampling.