In this hospital-based case-control study, children attending Siriraj Hospital and Queen Sirikit National Institute of Child Health from 1 December 2002 to 30 June 2003 were studied to define factors associated with TB in BCG immunized children (n = 260). Subjects of the same age and sex were divided into case and control groups by tuberculosis status. Caregivers were interviewed with a structured questionnaire. Data were analyzed by univariate analysis and multivariate analysis for biological factors (birth weight, health status, nutritional status), socioeconomic factors (parental education, education of caregiver, parental occupation, household incomes, and stability of household incomes), and environmental factors (history of contact with a tuberculosis patient, housing ventilation, child's bedroom ventilation, biomass smoke, passive smoking, crowded family and crowded in child's bedroom). Our findings show that children who had contact with TB patients had a very high risk of tuberculosis, even though they were vaccinated at birth. The risks vary according to the closeness level: very close (OR 85.67, 95%CI = 11.33-647.79), close (OR 31.11, 95%CI = 3.93-246.22) and not close (OR 32.70, 95%CI = 4.18-255.94). In order to identify the effect of others variables, the data was reanalyzed only in the group with no history of TB patient contacts (n = 192). Living in a crowded family, which was reflected by an average of 5 or more persons per room, also increased the risk (OR 11.18, 95%CI = 2.35-53.20). The other factor that increased the risk for tuberculosis was passive smoking. Children who were exposed to passive smoking had a 9.31 times increased risk of getting tuberculosis (95%CI = 3.14-27.58). These findings suggest that the public health department must develop a TB surveillance system in high TB prevalence areas, and in high density communities, and encourage smokers in every family to avoid smoking near children. Latent tuberculosis treatment recommendations for TB control cluster, as set by the Bureau of AIDS/TB and STIs, must be implemented in all health centers and an effective TB control program must be reinforced.