In order to compare the incidence of 1st molar caries and incidence of secondary amalgam caries appearing on the same tooth, yearly changes of the carious incidence were obtained from the protocols made at school dental examination of 344 children at Semmaya and amalgam fillings of 579 school children at the same town were examined one year after treatment. The rate of intact teeth was decreased in constant rate as shown in the formula, logP=1.989-0.051 T, where P means remaining intact teeth rate, T means school year, and constant 0.051 may denote real incidence of the 1st molar of the given population. On the contrary, resistance against amalgam injury decreased with ageing of the children (P=1.76+32.271/T). The present findings may conclude that even if the resistance against amalgam attack may be ascribed to the known chemical and physical changes appearing after tooth eruption, but host resistance against carious attack may not be always parallel with such physical changes of the tooth. It was estimated that the cariogenecity of a population in a certain community would remain constant during fairly long period of time, and additionally, the constant incidence was obtained after similar mathematical treatment of prevalence rates surveyed from each school year class. The real incidence of another tooth was also estimable from the similar formula but involvement of more cariogenic factors would be considered. Finally incidence of the amalgam injury should be denoted by a function of ageing.