Dental caries is the most common childhood disease worldwide.Between 60 and 90% of children are affected, but the majority of dental decay remain untreated [1]. Childhood dental caries has been reported to be the most prevalent infectious disease in our nation. It is 5 times more common than asthma and 7 times more common than hay fever among 5 to 17 year old American children [2]. In 1996, oral disease resulted in 1.6 million days of school lost; an average of 3.1 day per 100 students in the United States [3]. In 2009 -2010, the prevalence of untreated decay and decay Background experience among 6 -8 year old children examined by National Health and Nutrition Examination Survey (NHANES) throughout the United States was 17% and 45% respectively [4]. Data from National Health and Nutrition Examination survey for 2011 -2012 reported that approximately 37% of children aged 2 -8 years in the US had experienced dental caries, and 14% of the same age group had untreated tooth decay in primary teeth. While 21% of children aged 6 -11 had experienced dental caries in permanent teeth and 6% had untreated decay in permanent teeth [5].Background: Significant Caries Index (SCI) was proposed by WHO to draw attention to those individuals with the highest caries score in each population. The aim of the study was to evaluate the prevalence of dental caries and untreated caries, using combined decay, filling for primary teeth (dft) and permanent teeth (DFT) and SCI indices.Methods: 144 School children aged 4-13 years old were examined in a mobile dental clinic as part of the school-based dental services program. Combined dft/DFT and Significant Caries Index were used to evaluate dental caries prevalence. Dental caries were recorded when cavitation was observed clinically or radiographically. The services provided to children included comprehensive diagnostic, preventive and restorative treatment. Results:The mean combined dft/DFT for all groups was 3.18. As the age increased, the mean decreased with no significant differences. The SCI for all age groups was significantly higher than combined dft/DFT. The caries prevalence for this school was 63. 9%. About 77% of children had untreated caries. Conclusion:The study reported high prevalence of caries experience and untreated caries. The SCI is designed to focus attention to high caries risk group; it is useful indicator and helps in targeting the preventive program for this group. IRB # 1819-49.