Pulmonary high-grade neuroendocrine carcinomas (HGNECs) have poor prognoses and require multimodal treatment, and interstitial lung disease (ILD) restricts sufficient treatment of patients with lung cancer. We aimed to clarify ILD's prognostic impact on pulmonary HGNEC, which has previously gone unreported. We retrospectively analyzed 53 patients with HGNEC who underwent resections at our department between 2006 and 2021 and evaluated the clinicopathological prognostic features, including ILD. The patients' mean age was 70 years; 46 (87%) were male, and all were smokers. Large-cell neuroendocrine and small-cell lung carcinomas were diagnosed in 36 (68%) and 17 (32%) patients, respectively. The pathological stages were stage I, II, and III in 31 (58%), 11 (21%), and 11 (21%) patients, respectively. Nine patients (17%) had ILD, which was a significant overall survival prognostic factor in a multivariate Cox proportional hazards regression analysis (p = 0.048), along with lymph node metastasis (p = 0.004) and non-administration of platinum-based adjuvant chemotherapy (p = 0.003). The 5 year survival rate of the ILD patients was 0%, significantly worse than that of patients without ILD (58.7%; p = 0.003). Patients with HGNEC and ILD had a poor prognosis owing to adjuvant therapy's limited availability for recurrence and the development of acute exacerbations associated with ILD.