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Infection by M. tuberculosis can be demonstrated by tuberculin testing. On sequential PPD some person show a marked increase in the size of their skin reactions which may not be due to recent or past tuberculous infection. This phenomenon (booster effect) can occur as soon as a week after the initial test and persist for as long as a year. Methods and Materials: The purpose of this research is studying the size of induration following repeated PPD tests (booster effect) that studied in children who came to our clinic or imbedded in pediatric unit and the group of student in grades 4 and 5 at elementary school (totally 80 cases) for one year. Results: Increase of induration in 25% of all (20 cases) was 5 mm or more and in 75% of all (60 cases) was less than 5 mm and between these two groups, differences was meaning full (P< 0.05). The increase of induration in 12.5% was 7 mm or more. Conclusion: Because there is not shown definite increase of induration (more than 5 mm) on sequential PPD in children, we must to consider to new infection.