Collagenous gastritis is a rare condition characterized by surface epithelial damage, subepithelial collagen deposition, and a lamina propria inflammatory infiltrate. Previous studies have proposed 2 clinicopathologic subtypes: (1) children (18 y of age or younger) presenting with severe anemia, nodular gastric mucosa, and isolated gastric disease; and (2) adults with chronic watery diarrhea that is associated with diffuse collagenous involvement of the gastrointestinal tract. However, notable exceptions exist. In fact, broad variability in clinical presentation, etiology, treatment and disease course has been reported. To better define the clinicopathologic features of collagenous gastritis, we have collected 10 pediatric and 21 adult cases and describe their clinical, endoscopic, pathologic, and follow-up findings. Both children and adults presented with similar clinical symptoms such as anemia (50%, 35%, respectively), epigastric/abdominal pain (50%, 45%), and diarrhea (40%, 55%). Concomitant immune disorders were identified in 2 (20%) children and 3 (14%) adults. Further, 7 of 17 (41%) adults were taking medications associated with other immune-related gastrointestinal diseases including olmesartan and antidepressants. Histologically, there were no differences between children and adults with collagenous gastritis in the location of gastric involvement, mean collagenous layer thickness, and prominence of eosinophils (P>0.05). Extragastric collagenous involvement was also seen with comparable frequencies in each cohort (44%, 59%). Follow-up information was available for 22 of 31 (71%) patients and ranged from 2 to 122 months (mean, 33.6 mo). Despite medical management in most cases, persistence of symptoms or collagenous gastritis on subsequent biopsies was seen in 100% of children and 82% of adults. Of note, treatment for 1 adult patient involved cessation of olmesartan resulting in resolution of both symptoms and subepithelial collagen deposition on subsequent biopsies. Contrary to prior reports, no clinicopathologic differences were identified among pediatric and adult patients with collagenous gastritis. Whereas collagenous gastritis remains an enigmatic condition, our findings suggest that immune abnormalities and medications, such as olmesartan, may be possible triggers. However, current treatment options have had limited success and, thus, highlight the need for improved therapeutic regimens.