Data mining of digital dental records provides possibilities for analysing the variation between dentists when diagnosing caries. A total of 71,317 male and 82,302 female subjects visited the health centres in Vantaa and Kemi during the 'digital era' (1994-2005). As subjects were classified as 'new patients' at the first examination, all re-examinations of the same subjects thereafter by the same dentist produced 'old patients'. A mean number of decayed surfaces (DS) was counted as a function of the age of the subject during the follow-up. The significance of the difference between old and new patients was determined by the Mann-Whitney test for each age cohort at the cross sections, and for the whole follow-up. Caries was seen to affect new patients more than the old ones in both health centres after the age of 20 years. The mean DS values were the same, reaching about 1 for new and old patients at the age of 15 years. The mean DS had a peak for new patients in both health centres at the age of 25 years and another peak around 45-50 years in Kemi. With a few exceptions there was a significant difference between the DS values of new and old patients at most cross sections and for the whole follow-up time. Evidently dentists examine new patients more carefully than their old patients. After the age of 18 years patients may have changed their dentists because they have finished the free-of-charge treatment period.