Eight patients who had iron deficiency anemia after partial gastrectomy were tested to determine the role of blood loss and impaired absorption of iron in the individual case. Significant intermittent blood loss was documented in seven patients, while decreased iron absorption was found in only five. In four patients, both these factors were additive in causing iron deficiency. In the single patient with decreased absorption of the test dose of iron without evidence of bleeding, the anemia responded rapidly to therapeutic doses of orally administered iron. These data stress the relative importance of blood loss as a causal factor in iron deficiency after gastric surgery.