RESUMEN
To analyze the prevalence of dental caries by means of the DMF-S index and verify their distribution on the occlusal, buccal, lingual, mesial and distal surfaces using the intrabuccal interproximal technique, this comparative study included a sample of 360 seven to twelve year old boys and girls, without distinction of race, from the low social-economic class who, since birth, have lived in the cities of Teresina and Barras located in the state of Piauí in Brazil. The average level of fluor in first; city's public water supply is 0.68 ppm, while the second city lacks this halogen. A statistical analysis of the results verified that children in Teresina presented a DMF-S index 26.10% less than the children in Barras. The "T" Test of Student was significant for the seven, eight, nine, 10, and 12 years olds and for the sample as a whole in Teresina as well as for the types of surfaces regardless of age and sex. Children in the same community showed no significant difference when analyzed according to sex. In the two communities, the annual increase of the DMF-S index for all ages shows the cumulative and progressive aspect of dental caries; however, the increase was less in the city with fluoridated water.