The incidence and prevalence of nontuberculous mycobacterial pulmonary disease (NTM-PD) have been increasing worldwide. The risk of NTM-PD may be higher in undernourished populations. We tried to elucidate the impact of body mass index (BMI) and its change on development of NTM-PD in this study.We performed a retrospective cohort study including South Koreans age >40 years who underwent biennial National Health Insurance System (NHIS) health checkups in both 2005 and 2009 or 2006 and 2010. We monitored eligible individuals from the study initiation date (NHIS health checkup date on 2009 or 2010) until the diagnosis of NTM-PD or December 31, 2017. Enrolled individuals were classified based on BMI at initiation date. We compared NTM-PD incidence per 100 000 person-years by BMI group as well as by BMI change by calculating hazard ratio (HR).A total of 5 670 229 individuals were included in the final analysis. Compared with the BMI <18.5 kg·m-2 group, the incidence of NTM-PD gradually decreased with increased BMI: adjusted HR (aHR) 0.38 (95% confidence interval [CI] 0.35-0.42) for BMI 18.5-22.9, 0.17 (0.15-0.19) for BMI 23-24.9, 0.1 (0.09-0.11) for BMI 25-29.9, and 0.01 (0.07-0.13) for BMI ≥30. A BMI decrease of ≥1 kg/m2 over 4 years increased the incidence of NTM-PD (aHR 1.08, 95% CI 1.01-1.16) whereas a BMI increase of ≥1 decreased the NTM-PD incidence (0.77, 95% CI 0.71-0.83).In conclusion, BMI was inversely related to development of NTM-PD and weight loss increased the risk of NTM-PD.