We have identified 8 cases of gastritis characterized by the presence of purple to black granular deposits in the superficial mucosa associated with marked reactive epithelial changes. In each case, the patient had taken OsmoPrep, a tablet form of sodium phosphate used for bowel preparation just before upper endoscopy and had undergone concurrent colonoscopy. Endoscopic findings ranged from normal gastric mucosa to severe inflammation, congestion, and friability. No other gastrointestinal sites were noted to contain the deposits or show similar mucosal injury. On initial histologic review, the deposits raised the differential diagnosis of elemental iron and mucosal calcinosis. However, none of the patients was noted to be taking iron supplements, and none had a history of renal disease or other cause of calcium dysmetabolism. Histochemical stains revealed the deposits were negative on Perls' iron stain (8/8 cases), positive on von Kossa stain (7/8 cases), and negative on Alizarin Red stain (8/8 cases)-a histochemical profile compatible with sodium phosphate but inconsistent with mucosal calcium. A crushed OsmoPrep tablet was subjected to processing and demonstrated similar histologic features and histochemical profile. In addition, biopsies of 20 consecutive patients who did not take OsmoPrep and who underwent concurrent endoscopy and colonoscopy were reviewed, and no deposits with similar histochemical profile were identified. In summary, we have characterized a unique form of gastritis associated with OsmoPrep use. Attention to clinical history and use of a select panel of histochemical stains allow for accurate diagnosis.