Criteria for scoring the caries attack on each surface of an affected primary tooth are described. The score for each carious surface ranges from 1 to 3. The score per tooth is a sum of the surface scores. The caries severity index (csi) is the mean of the scores of all carious teeth in the population examined. Three groups of children aged 5-6.5 yr were examined. In Jerusalem, with a water fluoride concentration of 0.4 ppm, 54 children had a csi of 2.99 and a defs of 10.19. In a semirural African town, Soshanguva, with a 0.2 ppm fluoride concentration, 103 children had a csi of 3.10 and a defs of 5.75. In a nearby rural African area with a water fluoride concentration well above the optimum, 86 children had a csi of 1.93 and a defs of 1.99. Interexaminer and intraexaminer reproducibility of the csi were good. The csi expresses the severity of the caries attack on affected teeth and gives additional information to that of the def indices.