The prevalence of dental caries of the deciduous teeth as well as the permanent teeth among children suffering from the Down syndrome has been reported to be lower than in normal children. However, the reason of the low incidence of caries in the Down syndrome is unclear. Although there are many reports about the low incidence of caries caused by factors related to the clinical findings, there are few reports about the biochemical characteristics of the teeth of patients with Down syndrome.<BR>This study aimed at making a comparative analysis of the biochemical composition before and after the neonatal line on deciduous teeth with the Down syndrome patients and in normal children.<BR>Mandibular deciduous central incisors without caries were extracted from 5 Down syndrome children(Ave. age 6.2±8.7) and 5 normal children(Ave. age 5.8±10.1) at a dental hospital in Nagoya, Japan.<BR>Fluoride, phosphorus and magnesium concentrations from the enamel surface across the neonatal line before the dentine were analyzed using the abrasive micro-sampling technique, as described by Weatherell et al.<BR>The differences in fluoride and the magnesium concentrations between children with the Down syndrome and the controls were evaluated using 2-way ANOVA and Scheffe tests.<BR>The following results were obtained;<BR>1) F concentration: Fluoride concentration was higher at the outermost enamel and decreasing towards the interior both in children with the Down syndrome and in normal children.<BR>The average fluoride concentration of in children with the Down syndrome was lower than in normal children.<BR>In particular, there was a statistical difference of 5% at the 40,80μm from the neonatal line in the enamel formed before birth, and 40μm from the neonatal line formed after birth<BR>2) Mg concentration: There were no changes from the outermost enamel toward the interior of shown in both the children with the Down syndrome and in normal children.<BR>The average magnesium concentration in children with the Down syndrome was higher than in normal children. However, all the fixed depths to be measured showed no significant differences.<BR>The above results were not able to prove the low incidence of dental caries among children with the Down syndrome. It was concluded that the mineralization level of the enamel was lower in children with the Down syndrome than in the control teeth. It seems that the influence of mineralization on the enamel continued from before to after birth.