Introduction: Baddini-Capelozzi?s Necrosis (BCN) is a histological feature observed in fibrosing interstitial lung diseases (F-ILDs), presenting as intraluminal, amorphous, and basophilic material resembling the necrosis seen in gastric ulcers. Its clinical significance remains uncertain. Aims and Objectives: To investigate the role of BCN in F-ILDs. Methods: We selected 25 lung surgical biopsies with a confirmed or strong F-ILD diagnosis? microaspiration-related interstitial fibrosis (MARIF), fibrosing hypersensitivity pneumonitis (f-HP), or connective tissue disease (CTD)?after multidisciplinary discussion (MDD) from 2017 to 2021. Intra-alveolar contents were classified as BCN or mucus, followed by proteomic analysis and digital PCR (dPCR). Results: BCN was associated with bronchiolar ulceration and adaptive responses, including squamous metaplasia and myofibroblastic proliferation. Proteomic analysis identified five distinct animal proteins, 2 with at least 5 peptides and all sequences longer than 24 amino acids, in 70% of patients with BCN. Additionally, dPCR detected animal DNA fragments of three genes in 60% of BCN cases, with validation group analysis showing animal DNA in 50% of cases. Conclusions: BCN emerges as a novel biomarker for MARIF in F-ILDs, significantly impacting diagnostic accuracy and clinical management, underscoring the role of pulmonary pathology in the comprehensive assessment of ILDs. Funding: FAPESP Nº22/02821-0, 21/09024-6, 21/10981-5, 19/01517-3, 2023/10186-6, and 23/04199-8.