Related to lymphocytic colitis, collagenous colitis, lymphocytic gastritis & celiac sprue and is probably part of the same disease spectrum.

-The patient  usually complains of watery diarrhea.
-Endoscopic features include nodular pattern with erythema of the gastric body.

[ Collagenous gastritis: a report of six cases. Am J Surg Pathol. 2001 Sep;25(9):1174-9.   " 1) collagenous gastritis occurring in children and young adults presenting with severe anemia, a nodular pattern on endoscopy, and a disease limited to the gastric mucosa without evidence of colonic involvement, and 2) collagenous gastritis associated with collagenous colitis occurring in adult patients presenting with chronic watery diarrhea. These findings highlight the fact that subepithelial collagen deposition may be a generalized disease affecting the entire gastrointestinal tract. "  ]

Histological features: 

- Subepithelial thickening  of basement membrane (greater than 10 microns).
- Focal superficial epithelial degeneration
- Dense lymphoplasmacytic infiltrate in the lamina propria.
- In some cases numerous intraepithelial lymphocytes may be present.

Abstracts:

Collagenous gastritis in a Korean child: a case report.
J Korean Med Sci. 2005 Feb;20(1):146-9.

Collagenous gastritis: a long-term follow-up with the development of endocrine cell hyperplasia, intestinal metaplasia, and epithelial changes indeterminate for dysplasia.Am J Clin Pathol. 2001 Nov;116(5):753-8.

Collagenous gastritis associated with lymphocytic gastritis and celiac disease.Arch Pathol Lab Med. 2001 Dec;125(12):1579-84.

Collagen gastritis, an unusual cause of anemia in children. Report of 2 cases.Arch Pediatr. 2001 Jan;8(1):47-50.

Collagenous gastritis: a case report, morphologic evaluation, and review.Mod Pathol. 2000 May;13(5):591-6

Collagenous gastrobulbitis and collagenous colitis. Case report and review of the literature.Scand J Gastroenterol. 1999 Jun;34(6):632-8

Collagenous gastritis revealed by severe anemia in a child.Hum Pathol. 1998 Aug;29(8):883-6.