Background: Tuberculosis is one of the leading causes of mortality and morbidity in the world, especially in developing countries. Tuberculosis can transmit from person to person through the air. Thus, household contacts are considered as the high risk group of being infected. To prevent and control tuberculosis, screening of high risk population is widely recommended. However, it is seldom practiced in developing countries because of lacking resources. In Vietnam, National Tuberculosis Program (NTP) focuses on detection and treatment of new tuberculosis cases aiming at exceeding the targets of World Health Organization (WHO) in controlling tuberculosis. Despite these efforts, Vietnam still ranks 13th out of 22 countries with highest burden tuberculosis worldwide. And there is no evidence of decreasing in the number of new tuberculosis detected cases in any reports from NTP. Epidemiology of tuberculous infection among children has not been emphasized by Vietnam National Tuberculosis Program. Additionally, all strategies of NTP have focused on detection and treatment of new cases rather than identifying risk factors for tuberculous infection among contacts.Tuberculin skin test , the only method of diagnosing tuberculous infection in Vietnam still has many drawbacks because of cross-reaction with BCG vaccination as well as environmental mycobacterium. The whole blood interferon gamma assay has been shown to be more specific than tuberculin skin test and can be used in diagnosing tuberculous infection by numerous studies. Almost all studies have been conducted in low incidence countries and rarely in high burden countries.Better understanding the epidemiology of tuberculosis among children contacts as well as having the best method of diagnosing tuberculous infection could be a valuable distribution to the evaluation of undergoing TB transmission in the community. We, therefore, conducted a study in three provinces in northern Vietnam in order to identify risk factors for TB infection among children contacts. Objectives: The study is to compare the whole blood interferon gamma assay with tuberculin skin test in diagnosing TB infection and to identify risk factors for TB infection among children of smear positive tuberculosis cases.Method and materials: A cross sectional study was performed. 128 smear positive tuberculosis cases and 208 children contacts under fifteen of age between July 2007 and January 2009 were recruited into study. Clinical data of tuberculosis cases were collected based on hospital records. All children contacts underwent tuberculin skin test and interferon gamma assay from 2 to 15 weeks after detection. Face to face interviews based on the questionnaire were conducted with mothers on the day of admission.Results: A total of 128 tuberculosis cases and 208 children contacts in Ha Noi, Ha Tay and Thai Binh provinces were recruited into study. Children contacts were under 15 (of age), 38% of which was under 5.The overall, agreement between TST positive (cut off point of 10 mm) and INF gamma assay positive was substantial (Kappa: 0.62; 95% CI: 0.45-0.75). In multivariate analysis, using generalized estimating equation model:Factors remained association with TST positive, namely have low income (OR=3.526; 95% CI 1.598-7.779; p=0.002); have parent with tuberculosis (OR= 7.913; 95% CI 2.886-21.696; p<0.001); exposed to sputum smear grade of 3+ (OR=3.098; 95% CI 1.134-8.461; p=0.027); exposed to female with tuberculosis (OR= 0.309; 95% CI 0.126-0.758; p=0.01). Factors remained association with INF gamma assay positive was: Duration of cough before treatment (OR=6.9; 95% CI 1.9-25.07; p =0.003), have low income (OR=6.142; 95% CI 2.055-18.358; p = 0.001); have parent with tuberculosis (OR= 4.458 95% CI 1.041-19.093; p=0.044) and exposed to sputum smear grade of 3+ (OR=11.313; 95%CI 1.972-64.903); p=0.006).Conclusion: In this study, we found that the proportion of TB infection among children contacts was similar with both two tests. The agreement between TST and INF gamma assay was substantial. The results from study suggested that the children contacts with AFB smear positive tuberculosis cases had high proportion of TB infection as determined by both two tests. Have low income; closed contacts with tuberculosis cases; having parents with tuberculosis; delay in tuberculosis diagnosis and the density of bacteria in sputum seem to contribute to the spread of M. tuberculosis infection. We recommend an awareness program to prevent TB infection among children contacts from tuberculosis cases in family. Contact investigation should be considered as part of National TB program.Key words: Tuberculous infection, risk factors, epidemiology, interferon gamma assay, children under fifteen of age, rural areas, Vietnam.