The association between body mass index and mortality in patients with tuberculosis has not been extensively studied, and the existing evidence is inconsistent. This study aimed to investigate the impact of body mass index on timing of death in patients with tuberculosis. All Taiwanese adults with tuberculosis in Taipei, Taiwan, were included in a retrospective cohort study in 2011-2012. Multinomial logistic regression was used to evaluate the association between body mass index and timing of death in patients with tuberculosis. Among 1557 eligible patients, 84.1% (1310), 8.2% (128), and 7.6% (119) underwent successful treatment, early death, and late death, respectively. The mean age of the patients with tuberculosis was 64.2 years old, and 67.7% were male. After controlling for potential confounding variables, underweight with body mass index less than 18.5 kg/ m2 was significantly associated with elevated risk of all-cause mortality Adjusted odds ratio (AOR), 1.64; 95% confidence interval (CI), 1.17-2.30. Considering timing of death, underweight with body mass index less than 18.5 was significantly associated only with elevated risk of early mortality within the first 8 weeks of treatment onset (AOR, 2.22; 95% CI, 1.45-3.40). For patients with tuberculosis infection, underweight with body mass index less than 18.5 kg/ m2 is an independent predictor for early mortality within the first 8 weeks of treatment.