Statement of Problem: The dental hard tissues might be destroyed by different factors. One is dental erosion defined as the progressive loss of hard dental tissues by a chemical process not involving bacterial action.Purpose: The purpose of this study was to determine the prevalence of dental erosion and its risk factors in 12-year-old school children.Materials and Methods: This cross-sectional study was carried out on 483 12-year-old school children in Mashhad, 2007. Dental erosion was recorded for the labial and palatal surfaces of the upper incisors. For measurement purposes, the O'sullivan's index was adopted, and the results were statistically analyzed by T-student and Chi-Square tests.Results: The prevalence of dental erosion was 38.1%, with no significant sex difference. Dental erosion was significantly higher in private school children (P<0.001) and in underprivileged areas (P=0.005). Matt appearance of the enamel was the most prevalent type of dental erosion (21.2% central incisors, 5.2% lateral incisors). In most of the involved cases, more than half of their surfaces were diagnosed as affected by erosion (24.4% central incisors, 5.2% lateral incisors). The frequency consumption of carbonated beverages and night drinks illustrated a significant relation with dental erosion (P=0.01, P=0.023). Children who swam professionally in swimming pools had significantly higher dental erosion (P<0.001).Conclusion: Based on the findings of this study, individually tailored preventive programs may be recommended to patients and a comprehensive case history should be taken so that all risk factors can be revealed.