Predicting future caries risk is a difficult but important clinical task. The aim of this study was to analyze radiographically the relationship between approximal caries (4d-7m) at ages 11-13 (baseline) and future approximal caries. We followed 534 individuals prospectively through annual bitewing radiographs from 11 to 22 years of age. Two measures were used: individual-based incidence of the first new approximal caries lesion and surface-based incidence of approximal lesions. In the group with no approximal caries lesions at baseline, the individual-based incidence was 19 first new approximal lesions/100 person-years; the corresponding value for those with 3 approximal lesions at baseline was 71. Individuals with no approximal lesions at baseline developed 3.1 new lesions/100 tooth surface-years; the corresponding value for those with 3 lesions at baseline was 7.7. The highest risk for developing new approximal lesions was within the first 2 years after baseline.