Infection by M. tuberculosis can be demonstrated by tuberculin"ntesting. On sequential PPD some person show a marked increase in the size of their"nskin reactions which may not be due to recent or past tuberculous infection. This"nphenomenon (booster effect) can occur as soon as a week after the initial test and"npersist for as long as a year."nMethods and Materials: The purpose of this research is studying the size of"ninduration following repeated PPD tests (booster effect) that studied in children"nwho came to our clinic or imbedded in pediatric unit and the group of student in"ngrades 4 and 5 at elementary school (totally 80 cases) for one year."nResults: Increase of induration in 25% of all (20 cases) was 5 mm or more and in"n75% of all (60 cases) was less than 5 mm and between these two groups,"ndifferences was meaning full (P< 0.05). The increase of induration in 12.5% was 7"nmm or more."nConclusion: Because there is not shown definite increase of induration (more than"n5 mm) on sequential PPD in children, we must to consider to new infection.