Background <p>Nontuberculous mycobacterial pulmonary disease (NTM-PD) is a chronic, debilitating infection that predominantly affects patients with a low body mass index (BMI) and leads to progressive destruction of lung parenchyma. Although pneumonia is a common cause of death among patients with NTM-PD, information on its prognosis is scarce. We evaluated the association between BMI and outcomes in patients with NTM-PD who were hospitalized for community-acquired pneumonia (CAP).</p> Methods <p>Using the Japanese Diagnosis Procedure Combination database, we identified patients with NTM-PD hospitalized for CAP between April 1, 2012, and March 31, 2020, prior to the coronavirus disease 2019 epidemic in Japan. We evaluated in-hospital mortality as the primary outcome, and length of hospital stay and readmission due to CAP within 1&#xa0;year as secondary outcomes. Restricted cubic spline regression models were used to evaluate the impact of BMI as a continuous variable on the study outcomes, adjusting for potential confounders.</p> Results <p>We included 13,302 patients (median age: 79&#xa0;years [interquartile range {IQR} 72–85]; women: 63.7%; median BMI: 17.6&#xa0;kg/m² [IQR 15.6–19.9]). The median hospital stay was 14 d (IQR 9–25), and in-hospital mortality was 10.9%. In the restricted cubic spline model, being underweight (&lt; 18.5&#xa0;kg/m²) was associated with a sharp increase in both in-hospital mortality and length of hospital stay. In the overweight group (25–30&#xa0;kg/m²), high BMI was not associated with adverse outcomes. Being underweight was also associated with a modest increase in readmission because of CAP within 1&#xa0;year.</p> Conclusions <p>These findings indicate that assessing BMI may play a critical role in stratifying risk and optimizing management of CAP in patients with NTM-PD.</p>