In 100 hospitalized older patients with iron-deficiency anemia, the mean corpuscular hemoglobin concentration (MCHC) varied with the degree of anemia, but the degree of poikilocytosis and cell size did not; in 21 patients mean corpuscular volumes (MCV's) were normal or above normal. Nucleated red blood cells (RBC's) were seen on the blood smears of 22 patients. Platelet counts often were considerably increased. Diameters of primitive bonemarrow erythroblasts were smaller than those of normal persons. Chronic, occult bleeding from the gastrointestinal tract was the usual cause of iron loss. More than 12 different gastrointestinal lesions were found in this group; no one type of lesion predominated. In 21 patients no source of blood loss could be found.