SETTING: Although diabetes (DM) and low body mass index (BMI) are established risk factors for active tuberculosis (TB), the joint effect of type 2 diabetes (T2D) and BMI is unclear.DESIGN: A prospective cohort of 63,257 adults aged 45-74 years were recruited from 1993 to 1998 in Singapore. Active TB cases were identified via linkage with the National TB Registry up to December 2014. Cox regression models were used to estimate the hazard ratios (HRs) and 95% confidence intervals (CIs) for the relations of T2D and BMI, independently and jointly, with TB risk.RESULTS: T2D was associated with increased TB risk (HR 2.31, 95% CI 1.93-2.78). Conversely, BMI was inversely associated with TB risk: HR for underweight (BMI < 18.5 kg/m²) was 2.87 (95% CI 2.15-3.82) compared to obese (BMI ≥ 27.5 kg/m²) individuals. Compared to obese individuals without T2D, HR for active TB among underweight individuals with T2D was 8.30 (95% CI 4.43-15.54). There was no statistically significant interaction between BMI and T2D on TB risk (Pinteraction = 0.85).CONCLUSION: Underweight and T2D are independent determinants for active TB. This has important public health implications in Asia where prevalence of tuberculous infection is high, and T2D occurs at lower levels of BMI.