Collagenous gastritis (CG) is a rare condition that primarily affects the digestive system. People with CG have increased buildup of collagen in the subepithelial layer of the stomach. This condition typically affects children and young adults up to 22 years, or older adults over 35 years of age. Signs and symptoms appear to vary depending on the age group. Initial symptoms in children and young adults often include anemia and abdominal pain, whereas older adults often have chronic watery diarrhea associated with collagenous colitis, celiac disease or both. Adult collagenous gastritis is also associated with autoimmune diseases such as Sjögren syndrome, lymphocytic gastritis, lymphocytic colitis, and ulcerative colitis. Other signs and symptoms of CG may include nausea and vomiting, weight loss, abdominal distention, and gastrointestinal bleeding. The cause of the condition is unclear. Because of the small number of cases, no standard therapy for CG has been established based on randomized, controlled clinical trials.

Because collagenous gastritis is very rare and its cause is unknown, there is currently no established standard therapy for the condition. Various therapies have been attempted with limited success. These have included anti-secretary agents, steroids, iron supplementation, and hypoallergenic diets. Several other therapies have also been tested. A few affected people have shown improvement of symptoms, but no randomized, controlled trials have been performed. More studies are needed to establish a standard treatment strategy.

In the majority of adults, the condition seems to follow a chronic, intermittent course, with no significant mortality risk or severe progression. Diarrhea may resolve with or without treatment, although relapses may occur. In children, a less controllable course has been suspected. However, CG in children seems to follow a generally benign course, with limited long-term morbidity and no increased mortality reported to date. A few reports have documented that the abnormal collagen band in affected people persists (with or without medication) despite symptoms improving.

Collagenous gastritis is a rare disorder first described in 1989. It has been reported that less than 50 cases of this condition have been published in the English language literature since it was first described.