<sec><title>BACKGROUND</title>Undernutrition is the leading risk factor for TB worldwide and is associated with an increased risk of unfavorable outcomes and mortality. In this study, we assessed the impact of undernutrition before TB onset on radiographic and microbiological markers of TB severity.</sec><sec><title>METHODS</title>The RePORT India Consortium enrolled individuals with confirmed pulmonary TB (2015-2019). We used this data to conduct a prospective cohort analysis. Primary exposure was moderate-severe undernutrition (body mass index [BMI] <17 kg/m²) before TB disease was calculated using anthropometric data and patient-reported weight loss. Outcomes included lung cavitation, high-grade sputum smear positivity (>2+), percentage of lung affected, and time to positivity in mycobacterial growth indicator tube testing. We used multivariable regression and generalised additive models to assess non-linear relationships between premorbid undernutrition and outcomes.</sec><sec><title>RESULTS</title>Of 1,587 participants, 226 (14.2%) were undernourished before TB onset. Premorbid undernutrition was associated with increased odds of cavitation (aOR 1.80, 95% CI 1.14-2.89) and greater lung involvement (4.6%, 95% CI 0.73-8.42). The Generalized Additive Model (GAM) analysis revealed a non-linear relationship between premorbid BMI and cavitation but not for other severity metrics.</sec><sec><title>CONCLUSIONS</title>Premorbid undernutrition significantly increases the risk of severe pulmonary disease in TB patients. This underscores the importance of nutritional interventions to mitigate TB severity and improve outcomes.</sec>.