A prospective study was undertaken to identify mucosal lesions that might cause chronic blood loss anemia in patients with large diaphragmatic hernia. Patients with one-third or more of the stomach above the diaphragm on barium x-ray were examined by a gastroscopist who was given no clinical information. A total of 109 patients were included: 55 with anemia and 54 with a large hernia but no anemia. The incidence of esophagitis and peptic ulcer did not differ significantly in the anemic and nonanemic groups. Linear gastric erosions were found on the crests of mucosal folds at or near the level of the diaphragm in 23 anemic patients and 13 without anemia (p less than 0.05). Blood on the surface of a linear erosion was found in 14 anemic patients and 4 without anemia (p less than 0.05). We suggest that these erosions are due to trauma and can cause chronic blood loss anemia in hernia patients.