From an analysis of serial bitewing radiograms, we have developed a mathematical model of the initiation and progression of approximal carious lesions in the permanent teeth. The model is used to estimate the expected number of lesions, per individual, not detected until they reach the inner half of the dentin, as a function of the frequency with which radiograms are taken between the ages of 8 and 20 years. If radiograms are performed every 6 months and lesions not restored until radiolucencies appear in the dentin, under 5% of all lesions developing over the 12-year period will reach the inner half of the dentin before detection. If radiograms are taken every 2 years, about 18% of all lesions will have reached the inner half of the dentin before detection. The sensitivity of these conclusions to different assumptions is examined.