Previous studies have indicated that the prevalence of marginal alveolar bone loss (MABL) in several populations, but not yet in the Mexican children. Therefore, the present study examined the prevalence of MABL in a group of Mexican children. The characteristics of the proximal surfaces and the status of the adjacent alveolar bone, were recorded at 1307 sites from 209 diagnostic bite-wing radiographs, from 115 children. MABL was found in the radiographs of 7.8% of the children or 1.5% of the sites. The present prevalence finding is relatively low when compared to previous studies on children attending dental clinics. MABL was found already in 4-year-old children, confirming previous studies, which indicate the possibility of early onset of periodontitis in children.