Nivolumab can cause interstitial lung disease (ILD), which may be fatal; however, mortality risk factors have not been identified. This post-marketing study evaluated the poor prognostic factors of ILD in nivolumab-treated patients with non-small cell lung cancer (NSCLC) in Japan. Clinical and chest imaging findings for each ILD case were assessed by an Expert Central Review Committee, and prognosis was evaluated by radiographic findings, including the presence/absence of peritumoral ground glass opacity (peritumoral-GGO). Poor prognostic factors were identified by univariate and multivariate Cox regression analysis. Of the 238 patients with nivolumab-induced ILD, 37 died. The main radiographic patterns of ILD were cryptogenic organizing pneumonia/chronic eosinophilic pneumonia-like (53.4%), faint infiltration pattern/acute hypersensitivity pneumonia-like (20.2%), diffuse alveolar damage (DAD)-like (10.9%), and nonspecific interstitial pneumonia-like (6.3%). The main poor prognostic factors identified were DAD-like pattern (highest hazard ratio: 10.72), ≤60 days from start of nivolumab treatment to onset of ILD, pleural effusion before treatment, lesion distribution contralateral or bilateral to the tumor, and abnormal change in C-reactive protein (CRP) levels. Of the 37 deaths due to ILD, 17 had DAD-like radiographic pattern, 3 had peritumoral-GGO, and 5 had a change in radiographic pattern from non-DAD at onset to DAD-like. Patients with NSCLC who develop ILD during nivolumab treatment should be managed carefully if they have poor prognostic factors such as DAD-like radiographic pattern, onset of ILD ≤60 days from nivolumab initiation, pleural effusion before nivolumab treatment, lesion distribution contralateral or bilateral to the tumor, and abnormal changes in CRP levels.This article is protected by copyright. All rights reserved.