This study was designed to validate the SiC index in a 12-year-old population, and also considers the level of the disease, expressed as DMFS index, with the aim of comparing the capability of the two indices for preventive and prognostic goals.Data from a previous study (Campus et al, 2001) based on 403 12-year-old subjects (205 females and 198 males) were reconsidered, and the SiC was calculated both on DMFT and DMFS. Several background factors were evaluated: classified as socio-economic levels (SOCFAM), Oral Hygiene Habits (OHH), Onset of Toothbrushing Habits (OTH), and gingival conditions expressed as the presence of plaque or calculus. Several regression models were built-up to estimate the dependence of each index--DMFT, DMFT (SiC), DMFS and DMFS (SiC)--on background variables. The four groups, picked out by the 66th percentiles on the ranking series of DMFT and DMFS following the SiC method were compared.Mean+/-standard deviation, median and percentiles (p25 - p75) were 5.5+/-2.1, 5 (p25 = 4 - p75 = 6) for DMFT (SiC) and 8.9+/-5.8, 8 (p25 = 5 - p75 = 11) for DMFS (SiC). Intrinsic variability in SiC groups was lower, but the distributions remained skewed. In the multiple regression procedure, using DMFT and DMFS scores as dependent variables, OTH, bleeding and calculus were statistically significant. OHH (p or = 66th percentile. The concordance between the two selected series was rather good (kappa = 0.82; 95%CI: 0.73 - 0.91). No association with background factors was found on the two discordant groups. However, a linear trend in proportions between the two groups across SOCFAM categories was observed (p = 0.027).The use of SiC may solve the problem related to skewed caries distribution. Nevertheless if only SiC is used, it can lead to a lack of relevant information especially in countries where high caries prevalence is still present.