Background <p>Collagenous gastritis (CoG) is a rare disorder defined by subepithelial collagen deposition in the gastric mucosa, with associated lymphocytic or eosinophilic infiltrates in some reported cases.</p> Purpose <p>Our case series seeks to describe the clinical course of CoG in adults after treatment and associated endoscopic and histopathologic changes over time, specifically overlap with eosinophilic gastritis (EoG).</p> Methods <p>This case series includes eight patients diagnosed with CoG as adults at a single, large, academic tertiary care center between 2020 and 2024. Patient data were obtained through a retrospective chart review.</p> Results <p>The mean age at diagnosis was 34&#xa0;years, 50% were female, and 75% identified as White. Most reported abdominal pain, dyspepsia, and a recent history of gastrointestinal bleeding. Notably, all the eight patients were found to meet a concurrent EoG diagnosis on secondary retrospective review of the initial index CoG diagnosis biopsies. The majority had symptom improvement with budesonide and/or proton pump inhibitor (PPI) treatment. An elimination diet helped improve symptoms for 50% of the patients. However, only 25% had confirmed histopathologic improvement in collagen deposition.</p> Conclusions <p>Our case series suggests an apparent strong association between adult-onset CoG and EoG, raising the possibility that they may in fact be part of a disease spectrum and that chronic eosinophilic inflammation progresses to collagen deposition. Given a lack of established treatment guidelines for CoG, our study suggests that in addition to PPI and steroid treatments, an elimination diet may also be beneficial along with other EoG therapeutic modalities.</p>