Purpose: The body mass index (BMI) before lung transplantation (LT) is a benchmark of the post-LT survival. The aim of the study is to determine the BMI inadequate for the post-LT survival.Methods: We examined the survival after LT in patients grouped into the following BMI categories: <18.5 kg/m2 (underweight), 18.5-24.9 kg/m2 (normal weight), 25-29.9 kg/m2 (overweight), and ≥30.0 kg/m2 (obese) according to the World Health Organization (WHO) criteria. A more detailed categorization was made for further evaluation of the underweight group: mild (17.0 ≤ BMI < 18.5 kg/m2) and severely underweight (BMI <17.0 kg/m2).Results: There was no statistically significant difference in the post-LT survival between underweight and normal-weight patients (5-year survival: 78.7 vs. 76.1%). Patients with BMI <17.0 kg/m2 had a worse prognosis than those with 17.0 ≤ BMI < 18.5 kg/m2 (5-year survival: 70.3 vs. 90.0%).Conclusions: Standard BMI categorization per the WHO criteria is inadequate for determining the post-LT survival, especially in underweight patients. For the nutritional evaluation of underweight pre-LT patients, BMI <17.0 kg/m2 should be used instead of BMI <18.5 kg/m2.