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The purpose of this hospital-based case-control study is to determine the effect of passive and active smoking on pulmonary TB in adults. The study subjects were 100 new pulmonary TB cases diagnosed at TB Division, and age-sex matched 100 non-TB cases from patients admitted to Taksin Hospital and healthy subjects who came for annual physical check-up at either the outpatient clinic of the TB division or Taksin Hospital, during May 2001 to October 2001. All subjects had blood tests and only persons who were HIV-negative, DM-negative and free of other lung diseases were included. Data were collected by direct interview using questionnaires. Multivariate analysis of cigarette smoking related to pulmonary TB in adults was performed. The factors related to pulmonary TB in adults were current active smoking regardless of passive smoking exposure. There was a significant association between early age at initiation of smoking and TB. Active (current + ex-active) smokers who started smoking at age 15-20 years had a higher risk of pulmonary TB compared to others (OR = 3.18, 95% CI = 1.15-8.77); as well as the long duration of smoking: persons who had smoked >10 years had a higher risk of pulmonary TB (OR = 2.96, 95% CI = 1.06-8.22). There was a relationship between pulmonary TB and the amount of smoking exposure. Those who smoked >10 cigarettes/day (OR = 3.98, 95% CI = 1.26-12.60) or >3 days/week (OR = 2.68, 95% CI = 1.01-7.09) had higher risk of pulmonary TB compared to non-smokers. Passive smokers who were exposed to tobacco smoke >3 times/week outside the home had a higher risk of pulmonary TB than those with exposure < or =3 times/week (OR = 3.13, 95% CI = 1.07-9.17). It was also found that the effects of passive smoking in the office and/or neighborhood were strong. Persons with such exposures had a higher risk of pulmonary TB than no exposure or exposure < or =3 times/week from either or both places (OR = 4.62, 95% CI = 1.68-14.98). Therefore, an effective anti-smoking campaign is expected to have a positive repercussion on TB incidence. Smoking cessation must be considered and promoted by all levels of health care providers.